CHAPTER XVIII 

 WEISMANNISM AND OTHER THEORIES 



The theory of heredity described in the foregoing chapters, though 

 resting on its own foundation of observational and experimental evidence, 

 shows in some of its features the influence of certain earlier speculative 

 hypotheses, particularly those set forth by Weismann. 1 Some of the 

 conceptions embodied in these hypotheses are consequently involved in 

 cytological and genetical discussions of the present day, and for this 

 reason we shall here outline their main points, briefly indicating wherein 

 our modern theory has advanced beyond them. 



Although conceptions of other types arose very early, many of the 

 hypotheses in question were based on the assumption that the phenomena 

 of heredity and development are the result of the activity of ultimate 

 living particles of ultramicroscopic size. Thus Herbert Spencer (1864) 

 built up a theory of considerable proportions about his 'physiological 

 units/ and these formed the prototype of the units postulated in many 

 later theories. Of these theories the most prominent were those of 

 Darwin, Nageli, de Vries, and Weismann. 



Darwin's Hypothesis of Pangenesis. — In his Variations of Plants and 

 Animals under Domestication (1868) Darwin included a chapter on his 

 " Provisional Hypothesis of Pangenesis," which, though offered only as a 

 suggestion, excited great interest in the field of biology, especially after 

 the advances made in cytology a few years later. In several points it 

 closely resembled a theory propounded by Buffon more than a century 

 earlier (1749). Darwin clearly saw in the cytological aspects of heredity 

 one of the great biological problems of the future, but his only specula- 

 tions on the subject were embodied in the pangenesis hypothesis, which 

 may be stated as follows : 



All the cells of the organism at all stages of development give off small 

 particles, or gemmules, which multiply by fission and circulate throughout 

 all parts of the body. These gemmules pass to the germ cells, carrying 

 with them the power to reproduce cells like those from which they came. 

 In this way units representing all the kinds of cells composing the organ- 

 ism are collected in the gametes (or spores or buds) and are thus passed 

 on to the next generation. During the embryogeny of the new individual 

 the gemmules are so distributed that at the proper times and places they 



1 See Kellogg (1907), Delage and Goldsmith (1913), Thomson (1899, 1913) and 

 Conklin (1915). 



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