46 THE CAUSATION OF DISEASE. 



are developed by a single scale " of the leaf or stem ; and 

 finally lie observes that sperm- and germ-cells " differ from the 

 rest mainly in not having undergone modifications such as 

 those by which the rest are adapted to particular functions."* 



Finally, the question occurs : How conies it that a single cell, 

 -or the union of two cells, is able to reproduce the entire 

 organism ? Darwin and Spencer have started hypotheses of a 

 kindred character. The hypothesis of the former goes by the 

 name of Pangenesis. It is now universally admitted that all 

 organisms consists of cells, and that the latter are themselves 

 separate, and more or less independent organisms. Each cell 

 Darwin supposes to throw off tiny particles, or gemmules, which 

 are capable of reproducing the particular cells from which* they 

 are derived, and he accounts for the various methods of asexual 

 reproduction by supposing these gemmules to be scattered 

 throughout the body. Thus, when the entire tree grows from a 

 small twig, it is because that twig contains gemmules derived 

 from each separate cell in the entire plant ; and in a similar way 

 he would explain the development into entire animals of each 

 of the many fragments into which a Hydra may be divided. 

 The re-growth of an amputated limb he considers to be " the 

 same process partially carried out." In organisms which are 

 reproduced sexually, these gemmules are supposed to aggregate 

 themselves into the sexual elements — in the case of man, 

 namely, into the ovum and spermatozoon. Suppose, for 

 instance, an organism to cousist of homogenous, gelatinous 

 matter, then a small part separated from the rest would be able 

 io reproduce the whole. But if the upper, central, and lower 

 portions of the organism be of different textures, each would 

 have to throw off a different kind of gemmule, and these, 

 " when aggregated by mutual affinity, would form either buds 

 or sexual elements." There is no difficulty in allowing this 

 origin of gemmules from all the many cells of an organism, but 

 it is more difficult to understand why the gemmules should 

 aggregate themselves in proper order, why, for example, in the 

 case of germ and sperm, the countless gemmules for brain, liver, 

 lung, and other tissues, should be arranged according to a 

 definite plan. Darwin assumes a certain " elective affinity," in 



* "Principles of Biology," vol. i. § 77. 



