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 EVOLUTION

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EVOLUTION

formerly in a more perfect condition, so as to perform their typical functions — e. g., the eyes of the mole as organs of sight; and the limbs of the kiwi as means of locomotion for running or even for flying. Hence those individuals which now possess rudimentary or- gans are descended from ancestors which were in pos- session of these same organs in a less degenerated con- dition. But it cannot be ascertained from the struc- tures whether tho.se ancestors were of another kind than their offspring. The vermiform appendix in man is fully explained by supposing it to have had in antediluvian man a more perfect function of secretion, or even of digestion. Until the palEeontological rec- ords furnish us with more evidence we can only con- clude from the occurrence of rudimentary structures that in former ages the whale possessed better devel- oped limbs, that the moles had better eyes, the kiwi wings, etc. In short, rudimentary organs per se do not prove more than that structures may dwindle away by disuse.

Haeckel's endeavour to invalidate the teleological argument has no foundation in fact. In many cases the function of rudimentary organs has been discov- ered — e. g., the rudimentary teeth of the whale are probably of use in the growth of the jaw; the breast- bone of the slow-worm as a protection of the chest. But even in instances in which we have not succeeded in discovering the function of such structures, it must not be forgotten that degeneration may be eminently teleological in furnishing material for other organs whose functions become more important. Moreover, as long as rudimentary organs remain, they may be- come, under altered circumstances, the starting-point for an appropriately modified reorganization. It is indeed difficult to see how "dysteleology ", as Haeckel calls it, follows from the fact that an organ adapted to specified means of livelihood disappears, proljably in order to strengthen other organs when those means of livelihood are changed; and, until the contrary is proved, we may assume that we have to deal with in- stances of teleological adaptation and correlation, as has already been demonstrated in many cases — e. g., in the development of amphibians.

VI. The Ontogenetic Argument. — Comparisons between the embryos of higher forms and the adult stages of lower groups were made long before the evo- lution theory was generally accepted by biologists. But it was only after 1S5!) that the facts of embryology were interpreted by means of that theory. Fritz Miillcr (1.S64) was one of the first to advance the view that the ontogenetic development of an individual is a short and simplified repetition of the stages through which the species had passetl. Haeckel mollified the proposition by introducing the^term "kenogenesis", which should account for all points of disagreement between the two series of development. In its new form the theory of recapitulation received the name "the biogenetic law of development". Later on Hertwig reformed the law a second time by changing the expression "repetition of forms of extinct ances- tors", into "repetition of forms necessary for organic development and leading from the simple to the com- plex ". Besides, considerable changes, generally in an advancing direction, are said to have been brought about by the action of external and internal factors, so that in reality " a later condition can never correspond to a preceding one". Both Haeckel's and Hertn;ig's views were rejected by Morgan, who does not believe in the recapitulation of ancestral adult stages by the embryo, but tries to show that the resemblance be- tween the embryos of higher forms might be due to "the presence in the embryos of the lower groups of certain organs that remain in the adult forms of tliis group". Accorfling to Morgan, we are justified in comj)aring "the embryonic stages of the two groups" only — a theory which he calls " (he repetition lhef)ry".

Perhaps the most striking fact to illustrate the onto-

genetic argument is the resemblance between the gill- system of fishes and certain analogous structures in the embryos of the other vertebrates, man included. However, contrary to the statements of most scien- tists, we do not think that the resemblance is such as to justify us in concluding " with complete certainty that all vertebrates must in the course of thei'' history have passed through stages in which they were gill-breath- ing animals" (Wiedersheim). The embryos of fishes are at a certain very early stage of development fur- nished with vertical pouches which grow out from the wall of the pharynx till they fuse with the skin. Then a number of vertical clefts (gill-slits) are formed by the fact that the walls of the pouches separate. In the adult fishes the corresponding openings serve to let water pass from the mouth through the gill-slits, which are covered by the capillaries of the gill-fila- ments. In this way the animal is enabled to provide the blood with the necessary oxygen and to remove the carbon dioxide. Now it is quite true that in all vertebrates there is some resemblance as to the first formation of the pouches, the slits, and the distribu- tion of blood-vessels. But it is only in fishes that real gill-structures are formed. In the other vertebrates

the development docs not proceed beyond the forma- tion of the apparently indifferent pouches which never perform any respiratory function nor show the least tendency to develop into such organs. On the con- trary, the gill-slits and arches seem to have, from the very beginning, a totally different function, actually subserving, at least in part, the formation of other organs. Even the amphibians that are furnished with temporary gills form them in quite a peculiar manner, which cannot be compared with that of fish- embryos. Besides, the distribution of blood-vessels and the gradual disappearance of seemingly useless structures, as the "gill-systems" of vertebrates seem to be, may likewise be observed in cases where no one would seriously suspect a relation to former specific characteristics. In short, there is (1) no evidence that the embryos of mammals and birds have true incipient gill-structures; (2) it is probable that the structures interpreted as such really subserve from the very beginning quite different functions, perhaps only of a temporary nature.

In general it may be said that the biogenetic law of development is as yet scarcely more than a pclilio prinrijiii. Because (1) the agreement between onto- geny and phylogcny has not been proved in a single instance; on the contrary — e. g., tlie famous pedigree of the horse's foot liegins ontogeiietically with a single digit; (2) the oiilngenetic similarity which may be ob- served, for instiince, in the larval stages of insects may be explained by the similarity of the environment; (3) the ontogenetic stages of organisms are throughout specifically dissimilar, as is proved by a careful con- crete comparison. The same conclusion is iadicateH