42 THE PKINCIPLES OF HEREDITY 



more by light than by temperature. Thus animals and 

 plants which live in darkness are colourless, and white men 

 become bronzed under a fierce sun. 



70. A constant source of error arises from the practical 

 difficulty of distinguishing between modifications and varia- 

 tions. Thus when Nageli removed Alpine plants from their 

 natural habitat into the botanical gardens at Munich the 

 "species were so altered that they could scarcely be 

 recognized . . . but when such plants or even their descend- 

 ants were removed to a poor gravelly soil the new characters 

 entirely disappeared. . . . The retransformation was always 

 complete even when the species had been cultivated in rich 

 garden soil for several generations." l This difference in in- 

 dividual acquirements, combined with a difference in the in- 

 dividuals who survive under the altered conditions, may easily 

 account for very many of the changes which have been attri- 

 buted to variations directly caused by climatic influences. It 

 is difficult to understand how bud variations can be due to 

 the influence of the environment. Apparently they do not 

 regularly occur under given climatic or other environmental 

 conditions. In the case of the nectarine, for instance, one bud 

 out of the millions in an orchard of peach trees may vary in 

 such an extreme and definite manner as to bear nectarines. 

 It is at least highly unlikely that the external influences 

 acting on that one bud can have differed so marvellously from 

 the influences acting on all the other buds as to produce the 

 change. On the contrary, it is more probable that a bud vari- 

 ation, however striking, is nothing more than an ordinary 

 variation which appears rather late in the life of the individual. 

 Some plants grown from slips have not altered under the in- 

 fluence of the environment, though separated from the parent 

 stem by centuries of time and great expanses of space. One 

 writer has published observations which indicate great 

 degeneration and a high rate of mortality among the children 

 of lead-workers. But his observations were not sufficiently 

 extensive to be conclusive, nor, apparently, have they been 

 corroborated. If correct they are very remarkable. It would 

 seem, then, that lead does not destroy the spermatozoa ex- 

 posed to its direct influence, but only their very remote cell- 

 descendants in the children's bodies, and that only long after 

 the direct influence of the lead has ceased. 



71. On the whole the evidence collected by biologists is 

 neither large in amount nor conclusive in character. Many 

 of the phenomena instanced by them are evidently capable of 

 1 Weismann's Essays, vol. i., p. 276. 



