PROFESSOR WEISMANJPS THEORIES. 481 



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 distrib- 

 uted over an unlimited nutritive area, so that they can get materi- 

 als for growth at no cost, and expend nothing appreciable in mo- 

 tion 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 con- 

 tinued without break. But now let us pass to analogous illustra- 

 tions 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 mollusks and fishes. Of 

 one of them we read : " Gyrodactylus multiplies agamically 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 transformed into smaller 

 sexless individuals, which severally, before or after their emer- 

 gence, undergo similar transformations — a multiplication of so- 

 matic cells without any sign of reproductive cells. Under what 

 circumstances do such modes of agamic multiplication, 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 in- 

 stances are furnished by groups in which, though the nutrition is 

 not abundant, the cost of living is almost unappreciable. Among 

 the Cozlenterata there are the Hydroid Polyps, simple and com- 

 pound ; and among the Mollusca we have various types of Ascidi- 

 ans, fixed and floating, Botryllidce. and Salpce,. 



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 main- 

 taining temperature, and the food, some of it supplied by the 

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

 bathes the outer surface : the utilization of its contained material 

 being effected gratis by the sun's rays. Just as was to be ex- 



* A Manual of the Anatomy of Invertebrated Animals, by T. H. Huxley, p. 206. 

 vol. xliii. — 33 



