Page:The New International Encyclopædia 1st ed. v. 07.djvu/376

* EVOLUTION. 328 EVOLUTION. growth and reproduction, but in producing varia- tion, and. by causing a change in habits, in bringing about profound modifications of the body. We will begin with the simplest organ- isms. Maupas believes that the reproductive power of ciliate infusoria depends (1) on the quality and quantity of food; (2) on the tem- perature; and (3) on tin- alimentary adaptation of the buccal organ. With favorable nutrition an infusorian ( 8 tylonichia pustulata) undergoes - elf division once in twenty-four hours at a tem- perature of 7° to 10° C. '(42° to 50° F.), twice at 10° to 15° C. (50° to 60° F.). thrice at 15° to 20° C. (60° to 69° F.). four times at 20° to ('. |69° to 76° F.), and five times at 24° to 27 C. (76° to 80° F.). Thus at a temperature of from 25° to 20° C. (76° to 80° F.) a single Stylonichia would in 7% davs have a progenv of 100,000.000,000, estimated 'to weigh 100 kilo- grams (about 230 pounds). With a vegetable diet the rate is much less and the size smaller. JIaupas also shows that infusoria continue to multiply by Sssion until the supply of food fails, when hunger leads them to conjugate. Polymorphism. It now appears that the poly- morphism of the social insects is due to the nature and amount of food. The existence of worker ants and bees, whose characteristics are not inherited from their parents, has been a stumbling-block to the theory of descent. As Darwin states it: "The difficulty lies in under- standing how- such correlated modifications of structure could have been slowly accumulated by natural selection"; the "acme of the diffi- culty" being "the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an al- most incredible degree, and are thus divided into two or even three castes." The castes, moreover, do not commonly graduate into each other, but are perfectly well defined; being as distinct from each other as are any two species of the same genus, or rather as any two genera of the same family. It lias also been found by Wheeler that in a Texan ant the several castes are at first all alike, the remarkable differences between the large and small-headed workers being due to differences in the amount and nature of the food. It is well known that the larvie of the worker honey-bees are fed with much less nutritious food than those of the queens, which are fed on 'pap' or •bee-milk.' a highly nitrog- enous food, which has apparently a singular power of developing the reproductive glands. The white ants (Termitidse) are remarkable I'm- the polymorphism of the species, there be- ing in one kind eight castes, among them workers with small heads, and others with large heads, and soldiers of two castes, i.e. small-headed and large-headed. It has been found by Grassi thai all these castes are born alike, and that the differences between the castes are chiefly to tin- varying nature of the food, and have nothing to do with heredity. The small-headed forms have a scanty diet, live on refuse matter. Rnd even eai their own excrement, this being used in the construction of their galleries. The sol- dier- live "ii sick or disabled companions. The young are fed only *itli saliva. From these facts it appears thai the amount and nature of the foi lu. i c "f i lie w onderful differ entiation of the castes of the social insects; be- sides this there are the results of the division of labor in the community, use and disuse, together with specialization of labor, arising from the varied life of the populous colony. Difference in the amount and nature of the food, involving low or under and high feeding, results in a discrepancy in the number of individ- uals of either sex. It is now clear that a preponderance in the number of females is the result of high or better feeding. Yung experimented on tadpoles and increased the proportion of females as the diet was improved. In the first brood, by feeding one set with beef, the percentage of females rose from 54 to 78; in the second set, fed with fish, it rose from 61 to 81; while in the third lot, when the especially nutritious flesh of frogs was supplied, •the percentage rose from 56 to 92, i.e. in the last ease the result of high feeding was that there ici,. 92 females to 8 males. The result of Dtising's experiments with sheep leaves little doubt that abundant moisture and food tend to the production of females, while high temperature produces males. The heavier well-fed ewes brought forth ewes, while the lighter, under-fed ewes gave birth to males. Giron divided a flock of 300 ewes into two equal parts, of which one-half were extremely well fed and served by two young rams, while the others were served by two mature rams and kept poorly fed. The proportion of ewe lambs was 60 per cent, and 40 per cent., respectively. Use axd Disuse as Factors. Thus far we have considered the action of those factors which are concerned rather with the origination of varieties and species than of higher types ; with the causes of specific variation rather than of the formation of genera, families, orders, classes, and branches or phyla. While gravity, light, and the allied factors evidently come into play in morphogenesis, the inquiry arises how the higher categories of organic forms originated. This must mainly have been accomplished through change of environment, inducing new needs, the formation of new habits, or change of function, all operating together and resulting in adapta- tions to the new mode of life. In all this the principle of use and disuse plays a most im- portant part. See Disuse. Xi:w Structures. An interesting example of the origin of a new structure due to change in habits and to resulting strains and movements is the formation of bivalve shells in animals of classes so unlike as the bivalve mollusks ( Pele- cypoda) and the ostracod, phyllopod, and phyl- locaridan crustaceans. The shell or carapace has become folded into two valves to protect tin 1 body. The valves are opened and closed by the relaxation or contraction of one or two peculiar muscles, the adductors. These muscles are net homologous with any muscles in other classes, and at least in bivalve mollusks thej are prob ably developed from the mantle muscle as a con- sequence of the conditions of the case. The sev- eral types thus occurring in different branches or phyla "is a strung proof that common forces acting on all alike have induced the resulting form" (Jackson). There are also good examples uf mimicry or 'convergence,' and many so-called eases of mimicry are undoubtedly merely ex amples of such convergence or similarity of form due to the subjection of animals of quite different groups to identical habits or conditions,