XV THE PHILOSOPHY OP ZOOLOGY 663 



germ-plasm in which they originate has a correspondingly com- 

 plicated and definite structure. 



The oosperm, having the faculty of reproducing the entire animal 

 without (in many cases) any further influence emanating from the 

 parent, must contain within itself something to represent each of 

 the parts — even each group of cells— of the adult body. The 

 oosperm of a Frog, for example (p. 288), simple though its structure 

 appears to be, must contain potentially within itself all the 

 characteristics of the adult animal, and not only these, but 

 the characteristics of each successive stage in the formation of 

 the tadpole and its metamorphosis into the adult Frog. Attempts 

 have been made to explain how it is that the reproductive cells 

 acquire this reproductive capacity. One of the most interesting 

 of these is a theory which is termed pangenesis, the origination of 

 which is due to Darwin. According to this theory, the cells of 

 the various parts of the body throw off minute ultra-microscopic 

 particles or "gemmules," and thes6 find their way by various 

 channels to the developing reproductive cells, in which they 

 accumulate until each reproductive cell contains gemmules repre- 

 senting all parts of the body. When development takes place 

 each gemmule develops into the part corresponding to that from 

 which it has been derived. 



If this theory afforded a true explanation of the facts of repro- 

 duction, there would necessarily be accumulated in the ovum 

 gemmules representing, not only every part of the body of the 

 adult, but also every stage in the development of the embryo, and 

 (since we see ancient ancestral characters occasionally reverted 

 to) something to represent the special peculiarities of former 

 generations. Now, it is a question if such an accumulation of 

 gemmules, each necessarily several times the size of a chemical 

 molecule, would not form a mass very much larger than an ovum. 

 Such a doctrine would, moreover, hardly appear to be necessary 

 in order to explain the facts. The accumulation in the ovum of 

 the hereditary tendencies (as we may call them) may only in part 

 take place during the life-time of the individual : a good part of 

 them — all, perhaps, except such as have been more recently 

 acquired — might be contained in the ready-formed germinal 

 material handed down from previous generations. 



Against a hypot?iesis of pangenesis such as was formulated by 

 Darwin, the mode of reproduction of many plants tells more 

 strongly perhaps than any of the facts derived from the animal 

 kingdom. Many of the higher flowering plants, for example, are 

 capable of being propagated by means of a cutting of the stem or 

 root, or even by a leaf. As the new plant developed from the 

 cutting gives rise to flower and fruit, the cutting must contain 

 germinal matter ; and germinal matter must, therefore, be diffused 

 throughout the cells of such a plant. Pangenesis, unmodified, 



