REPRODUCTION. 939 



ations of Animals and Plants under Domestication. It was the first of the 

 modern theories to attempt to cover the whole ground of heredity ; it was 

 accompanied by a most exhaustive presentation and analysis of facts, and it 

 stimulated abundant discussion and investigation. In Darwin's own words 

 the hypothesis was formulated as follows : " It is universally admitted that the 

 cells or units of the body increase by cell-division or proliferation, retaining 

 the same nature, and that they ultimately become converted into the various 

 tissues and substances of the body. But besides this means of increase I assume 

 that the units [cells] throw off minute granules which are dispersed throughout 

 the whole system ; that these, when supplied with proper nutriment, multiply 

 by self-division, and are ultimately developed into units like those from which 

 they were originally derived. These granules may be called gemmules. They 

 are collected from all parts of the system to constitute the sexual elements, and 

 their development in the next generation forms a new being ; but they are 

 likewise capable of transmission in a dormant state to future generations, and 

 may then be developed. Their development depends on their union with other 

 partially developed or nascent cells which precede them in the regular course 



of growth Gemmules are supposed to be thrown off by every unit, 



not only during the adult state, but during each stage of development of 

 every organism ; but not necessarily during the continued existence of the 

 same unit. Lastly, I assume that the gemmules in their dormant state have a 

 mutual affinity for each other, leading to their aggregation into buds or into 

 the sexual elements. Hence, it is not the reproductive organs or buds which 

 generate new organisms, but the units of which each individual is composed. 

 These assumptions constitute the provisional hypothesis which I have called 

 Pangenesis." 



Since the cells of the body are represented by gemmules within the germ- 

 cells, Darwin's theory is a theory of Preformation. It explains the facts of 

 the regeneration of lost parts by the assumption that the gemmules of the part 

 in question are disseminated throughout the body and have only to unite with 

 the nascent cells at the point of new growth. Pangenesis explains reversion, 

 since gemmules may lie dormant in one generation and develop in the next. 

 It explains congenital variation, since the mixture of maternal and paternal 

 gemmules is plainly different from the two kinds taken separately. It explains 

 how acquired variations may become congenital, since an altered part throws 

 off altered gemmules, and by the collocation of these in the germ-cells the 

 alteration may be transmitted. It thus allows the transmission of acquired 

 characters. 



Darwin's assumptions of gemmules and their behavior are pure assump- 

 tions, for which subsequent investigation has not provided a basis of facts. 

 As we have seen, also, the inheritance of acquired characters is greatly in 

 doubt, and, if they are heritable at all, they can be so only comparatively 

 feebly. Besides these objections it was early found that, with the increase 

 of knowledge of the facts of heredity, it was necessary to modify very mate- 

 rially the theory of Pangenesis. This has been ably done successively by 



