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THE AMERICAN NATURALIST [Vol. XLIV 



ous character of any explanation offered by the reaction 

 theory seems more in accord with the modern spirit of 

 scientific theory. But when we lay aside these generali- 

 ties concerning the two theories and descend to particu- 

 lars we may find at times very real distinctions between 

 the two views. For example : — 



The original conception of preformation postulated an 

 actual material embryo in the egg; epigenesis denied 

 the existence of that embryo, and justified its denial. 

 Here surely there was a real distinction. 



But the problem has refined itself in modern times. 

 We no longer look for an actual embryo preformed but 

 we look for samples of each part, which samples by in- 

 creasing in size and joining suitably to other parts make 

 the embryo. This is modern preformation. Is it not a 

 question of fact whether such samples exist in the egg? 

 The contrasting theory looks upon the germ-cells as 

 consisting of one fundamental material, or at most of a 

 few materials that change as development proceeds, until 

 finally the end-product of the changes are the kinds of 

 materials that we know to differ chemically in a number 

 of ways. It seems to me that there is here also a real 

 difference between the Uyo views, and that the one can 

 be as clearly formulated as the other: I propose, there- 

 light of our present opinions concerning the egg and its 

 mode of development. 



The modem theory of particulate inheritance goes 

 back no further than the discovery that the sperm trans- 

 mits equally with the egg the characters of the race; 

 and with the discovery that the most conspicuous thing 

 that the sperm brings into the egg is the nucleus of the 

 male cell or more specifically its chromatin. Around 

 these simple statements the whole edifice has been 

 erected. We owe to Weismann more than to any other 

 biologist, the peculiar trend that this speculation has 

 followed. It has seemed to many biologists that the only 

 interpretation of the facts just stated could be that 

 special turn that Weismann has given to them. 



