INADEQUACY OF NATURAL SELECTION, ETC. 041 



course I do not cite this in proof of the extent to which multipli- 

 cation of somatic cells, descending from a single ovum, may go ; 

 because it will be contended, with some reason, that each of the 

 sexless Aphides, viviparously produced, arose by fission of a cell 

 which had descended from the original reproductive cell. I cite 

 it merely to show that when the cell-products of a fertilized 

 ovum are perpetually divided and subdivided into small groups, 

 distributed over an unlimited nutritive area, so that they can 

 get materials for growth at no cost, and expend nothing appre- 

 ciable in motion or maintenance of temperature, cell-production 

 may go on without limit. For the agamic multiplication of 

 Aphides has been shown to continue for four years, and to all 

 appearance would be ceaseless were the temperature and supply 

 of food continued without break. But now let us pass to analo- 

 gous illustrations of cause and consequence, open to no criticism 

 of the kind just indicated. They are furnished by various kinds 

 of Entozoa, of which take the Trematoda, infesting molluscs and 

 fishes. Of one of them we read : " Gyrodactylus multiplies 

 agamicaliy by the development of a young Trematode within 

 the body, as a sort of internal bud. A second generation appears 

 within the first, and even a third within the second, before the 

 young Gyrodactylus is born." * And the drawings of Steenstrup, 

 in his Alternation of Generations, show us, among creatures of this 

 group, a sexless individual the whole interior of which is trans- 

 formed into smaller sexless individuals, which severally, before or 

 after their emergence, undergo similar transformations a multi- 

 plication of somatic cells without any sign of reproductive cells. 

 Under what circumstances do such modes of agamic multiplica- 

 tion, variously modified among parasites, occur ? They occur 

 where there is no expenditure whatever in motion or maintenance 

 of temperature, and where nutriment surrounds the body on all 

 sides. Other instances are furnished by groups in which, though 

 the nutriment is not abundant, the cost of living is almost un- 

 appreciable. Among the Coelenterata there are the Hydroid 

 Polyps, simple and compound ; and among the Mollusca we have 

 various types of Ascidians, fixed and floating, Botryllidce and 

 Sal pee. 



But now from these low animals in which sexless reproduction, 

 and continued multiplication of somatic cells, is common, and 

 one class of which is named " zoophytes," because its form of 

 life simulates that of plants, let us pass to plants themselves. In 

 these there is no expenditure in effort, there is no expenditure in 

 maintaining temperature, and the food, some of it supplied bv 

 the earth, is the rest of it supplied by a medium which every- 



* A Manual of tJie Anatomy of Invertelra'.ed Animals, by T. H. Huxley, 

 P. 206. 



