IN THE THEORY OF NATURAL SELECTION. 319 



velopment of an embryo is in almost all animals far smaller than 

 the amount provided by the female parent. 



In Mammalia the share contributed by the father probably only 

 forms about one hundred-billionth part of that contributed by the 

 mother, and yet nevertheless the influence of the former in he- 

 redity is on an average equal to that exerted by the latter 1 . Now, 

 from the point of view of epigenesis, no molecule of the brain of 

 an epileptic animal can reach the germ-cell except in a state of 

 solution, and therefore no direct increase in the germ-plasm can 

 be referred to such molecules, quite apart from the fact that such 

 addition, even if possible, could not be of any value, because the 

 last stage of the epileptic tendency must be represented in the 

 nerve-cells and nerve-fibres of the diseased brain, while the first 

 stage ought to be represented in the germ-cell. 



It may be safely asserted that according to the theory of epigenesis 

 the germ-cells cannot be influenced except as regards their nutri- 

 tion. Nutritive changes may be imagined to occur through the 

 varying trophic influence of the nervous system upon the sexual 

 organs, but the structure of the germ -plasm cannot be altered by 

 mere nutritive changes, or at all events it cannot be altered in 

 that distinct and definite direction which is required by the sup- 

 posed transmission of acquired epilepsy. 



Thus the transmission of artificially produced epilepsy can neither 

 be explained upon the epigenetic theory, nor upon the theory of 

 preformation ; it can only be rendered intelligible if we suppose 

 that the appearance of the disease in the offspring depends upon 

 the introduction and presence of living germs, viz. of microbes. 

 The supposed transmission of this artificially produced disease is 

 the only definite instance which has been hitherto brought forward 

 in support of the transmission of acquired characters. I believe 

 that I have shown that such support is deceptive, not because there 

 is any uncertainty about the fact of the transmission itself, but 

 because it is a transmission which cannot depend upon heredity, 

 and is in all probability due to infection. 



Ever since I began to doubt the transmission of acquired cha- 

 racters, I have been unable to meet with a single instance which 

 could shake my conviction. There were many instances in which 

 hereditary transmission was clearly established, but in none of them 



1 Niigeli, 1. c. p. no. 



