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same environment, became still yellower than their parents until the black pigment had been almost entirely displaced; whilst the offspring of two which had become darker, if reared in cages with black walls and floor, became practically completely black by the time they reached maturity so that they came to resemble the mountain species Salamandra atra. If, however, the offspring of two salamanders reared in yellow surroundings were allowed to grow up under black surroundings, they nevertheless for the first six months of their lives became progressively yellower; then and only then did the influence of the black environment begin to tell the yellow patches became invaded by numerous small black spots and grew smaller. In short, the young recapitulated the process of " yellow- ing " that their parents had undergone.

If these results are confirmed the doctrine of recapitulation will change its status from that of an hypothesis to that of a proved fact ; and further proof will be furnished that changes acquired by the individual in response to the demands of the environment are to a certain extent at least inherited.

The Recapitulation Theory. Once we have grasped the mutual relationship of the embryonic and larval phases of development, indirect proofs of the reality of recapitulation begin to crowd in on us. If we find, for instance, one or two aberrant forms in an order or even a family the majority of whose members have a uniform type of structure, no reasonable doubt can exist that the an- cestors of these aberrant forms had the typical structure of the group. If this conclusion be admitted and we find that the young- er stages of the aberrant species also show the typical structure, does any one seriously question that these young forms recapitu- late the history of the race? Two very striking instances of this kind have come to light within the group Ctenophora.

The typical Ctenophora are ovoid organisms of a glassy trans- parence which swim in a vertical position in the sea. Their locomotor organs are eight vertical rows of vibratile combs, each comb consist- ing of a short horizontal row of powerful cilia fused together at their bases. A certain creeping organism resembling a flat worm, named Coeloplana, had been believed by some zoologists to exhibit cteno- phore affinities but its relationships were very obscure. Quite recently a Japanese zoologist 1 has described its development. Its larva is a small typical ctenophore with eight rows of perfectly formed combs ; these it discards after swimming for a few hours it sinks to the bottom and flattens put and gradually assumes the adult structure. Another extraordinary organism, named by its discoverer Tjalfjellia, 2 was discovered amongst dredgings collected in the Arctic Ocean. This creature superficially resembled a sponge or an ascidian. It was gelatinous and sessile and seemed to consist of a pair of upright tubes like towers whence proceeded smaller tubes which ramified in its substance. In pockets connected with these smaller tubes were discovered groups of the larvae. These were small ovoid creatures of typical ctenophore structure with the eight vertical rows of combs.

If recapitulation of ancestral history forms an unquestionable element in the life history of some animals, is it not probable that it constitutes a factor in all life histories ? To this question it seems to us only an affirmative answer is conceivable.

Change of Habits. If we then regard the reality of recapitula- tion as proven we may now reflect on its meaning. We have seen that the recapitulatory element is most obvious in the latest larval stage of development, the most recently added page of the life history. Now the organs of the larva are adapted to its environment; therefore this environment in its broad outlines at least must represent the ancestral environment .of the race.

The present condition of the race both as regards structure and habits has been produced as a consequence of migration from the original haunts of the race. Change of habits therefore re- veals itself as the great driving-force in evolution, and change in habits usually means the choice of a different type of food.

We may conclude that the period of life at which this change most frequently occurred was when the adult organs had de- veloped but before sexual maturity had been attained in a word, at the stage of what we may call the young adult. As one change of habits succeeds to another in the course of evolution, the life history is not lengthened in the same proportion, since the new phase takes the place of the sexual phase in the previous condition of the race. In some Crustacea, e.g. in the shrimp Penaeus, at least four larval stages are passed through before the

1 Taku Komai, " Notes on Coeloplana bocki and its development," Annotationes Zoologicale Japonenses, vol. ix., 1920.

1 Mortensen, " Ctenophora," Danish Ingolf Expedition, vol. v., No. 2, 1912.

adult stage is attained, but in the majority of life histories when a new phase is added there is a tendency for some of the older phases to be pushed back into the embryonic period, so that as an animal passes from stage to stage in evolution it leaves behind a trail of stages at first larval and then becoming embryonic.

Secondary Modifying Factors. We may now glance at the principal factors which modify and tend to obscure the re- capitulatory factor. It is only possible to define these factors by a truly comparative embryology based on a wide survey.

One of these factors is " tachygenesis " or precocious development ; that is to say, we find that organs originally developed as a response to the stimulus of a new environment come in course qf time to be developed before the habits to which they correspond can be ex- ercised in fact acquired habits tend to become innate. Thus the young hermit crab when adult thrusts its abdomen into the cavity of a spirally coiled gastropod shell, and in this way imposes a twisted form on this part of its body. But if all such shells be removed from the hermit crab's neighbourhood at the time of its metamorphosis, it will still develop a curved abdomen although the extent of the curvature will be less than that which occurs normally. When the tadpole of the frog acquires limbs, these do not develop in the form of fins from which they have been undoubtedly evolved,, but grow directly into the ordinary type of five-toed limbs, although weeks must elapse after their form is fully defined before they can function as the limbs of land animals. The tendency to hurry on development may be compared to the increasing facility with which a difficult operation is performed after long practice, but this tendency obvious- ly tends to obscure the distinctive features of early development.

A second powerful modifying factor is the change from the larval to the embryonic phase, so far as the development of a particular organ is concerned. This change of phase is sometimes caused by an unfavourable alteration in the environment of the larva. It was actually effected artificially in the development of Salamandra maculosa by Kammerer. 3 This species is viviparous and normally gives birth to between 30 and 40 young which are provided with gill- slits and long gills and which live in the Water for six weeks before they metamorphose into land animals. If the parents are exposed to successively colder and drier conditions, the number of young produced at a birth diminishes with each breeding-period, and these young are born at a progressively more advanced stage of develop- ment. If these young are reared to maturity under similar conditions of coolness and dryness, they will in turn give birth to young which will be still fewer in number than those produced by their parents and which are born at a still more advanced stage of development. The process goes on till only three or four are born at one time and these are provided with the merest stumps of gills ; such young never enter the water at all but at once take up the adult mode of life. This is the normal mode of development of Salamandra atra.

The change of phase from the larval to the embryonic type entails many other changes. The embryo must be fed and it obtains its food from one of three sources, (a) devouring its sisters ; (b) secretions from the mother's womb ; (c) inclusions of yolk in its own cytoplasm. When the embryo devours its own sisters, this, as in the case of Salamandra atra, may entail little change of structure because the habit is one recently acquired ; but where, as in the case of the play- helminth worms, the habit is of old standing then the embryo may be distorted out of all recognition. In these worms one viable egg is shut up in a capsule along with thousands of small sterile ones ; and it is difficult to find in the embryo any vestige of resemblance to the larva of these Playhelminthes which lay their eggs singly.

When the embryo derives its nourishment from the mother's womb then it frequently develops organs of adhesion to the wall of this. To this category belongs the placenta which profoundly dis- torts the ventral surface of the human embryo, so that this surface gives rise to a treelike outgrowth whilst the dorsal surface is moulded into a ludicrously exact copy of the early tadpole of the amphibian.

When the embryo is fed by yolk, this, as we have already pointed out, modifies all the processes of development ; cell division becomes slow and the cells produced few and large, and folding which plays a large part in the development of small alecithal eggs becomes impossible and is replaced by solid outgrowths of cells.

Still a third factor which tends to hide the recapitulatory element is the development of special larval adaptations. This occurs when the larva retains its free life but when its circumstances become changed. These special adaptations have been developed in thou- sands of insect larvae. So generally is this the case that Half our* denied to these larvae any ancestral significance at all ; but modern research has succeeded in revealing the original ancestral larval type beneath the secondary modifications.

All the evidence at our disposal points to the conclusion that the ancestors of insects were creeping myriapod forms scavengers

3 Kammerer, " Vererbung erzwungener Fortpflanzungsanpas- sungen I & II. Die Nachkommen der spatgebprenen Salamandra maculosa und der fruhgeborenen S. atra," Archiv fur Entwicklungs- mechanik, vol. xxv., 1908.

4 Balfour, Comparative Embryology, vol. ii., p. 365, 1881.