RE PR OD UCTION. 501 



is one of the much discussed and still unsettled questions. At least two causes 

 of congenital variations are commonly recognized, although opinions differ as 

 to the relative importance of the role played by each. These causes are differ- 

 ences in the nutrition of the germ-plasm, and sexual reproduction. As to the 

 former, it is evident that the germ-plasm in no two individuals, even father 

 and son, has exactly identical nutritional opportunities. Since the life of one 

 individual is not the exact counterpart of the life of another, the germ-plasm 

 of one individual has a different nutrition from that of another. It would 

 hence be strange, even although we regard the germ-plasm as relatively stable, 

 if with succeeding generations there did not appear variations that are sufficient 

 to give rise to unlikeness in relatives. Differences in the nutrition of the germ- 

 plasm in different individuals are, therefore, a true cause of variations. As 

 regards sexual reproduction, it must be remembered that a new individual is 

 the product of two individuals, that the two individuals have descended along 

 different genealogical lines, and hence that the two conjugating masses of germ- 

 plasm are different in nature. It is only to be expected, therefore, that the 

 resulting individual shall be different from the two contributing parents. Thus 

 sexual reproduction is a true cause of variations. 



Having outlined the main facts and principles of heredity, let us now review 

 a few of the specific theories that have been of value in clearing the clouded 

 atmosphere. 



Darwin s Tlieory of Pangenesis. — Darwin's " Provisional Hypothesis of 

 Pangenesis" was published in 1868 as chapter xxvii. of his work on Tfie Vari- 

 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 arc 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 generation-, 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 tin-own oil' by every unit, 



not only during the adult stale, 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 



