464 Proceedings of the Royal Physical Society. 



generally believed that the condition of the reproductive 

 products, at the time of fertilisation, has some influence on 

 the sex, etc., of the offspring. And in regard to hybridisation, 

 which, for all we know, may have been of some importance 

 in the past, the recent experiments of the brothers Hertwig 

 demonstrate the importance of the condition of the elements. 



(3.) During the period of embryonic life, environmental 

 influences may effect very fundamental changes, whether 

 the embryo be within or outside the parent. The effect of 

 different nutrition on bee grubs at once suggests itself. By 

 varying the nutrition of his tadpoles, Yung was able very 

 widely to alter the normal proportions of the sexes. Ex- 

 periments on other young forms have been noticed above. 



11. Relation of Environmental Modification to Heredity 

 (223-246). — Till comparatively lately it was assumed that 

 characters acquired by an individual organism, and in no sense 

 part and parcel with its inherited constitution, might yet be 

 handed on to the offspring. Once acquired, the possibility of 

 their transmission was hardly doubted. Even Darwin, how- 

 ever, came to feel the great probability against the retention 

 of individual variations. Weismann has brouoht the doubt 

 to a climax, by a point-blank denial of the transmission of 

 individually acquired characteristics. This he does partly on 

 the ground that positive proof is in his opinion wanting, and 

 partly because, in most cases of individually acquired non- 

 inherent characters, there seems little reason to suppose 

 that the reproductive elements could have been affected. 

 This is an obvious sine qua 7ion of transmission. He allows, 

 indeed, that in the course of generations, if the conditions 

 of change persist, the variations will become more deeply 

 rooted ; the reproductive elements will somehow come to feel 

 the change ; and the modification will be transmitted, and for 

 a time, at least, stereotyped. In cases where the rudiment 

 of reproductive organs is set apart, and relatively isolated at 

 an early stage from the general somatic cells, Weismann's 

 caveat is especially impressive. It has been pointed out, on 

 the other hand, that no organs of the body are insulated ; the 

 blood is of course a common medium ; the various parts are, 

 to use a pedantic term, symbions ; and thus a modification of 



