GERM-PLASM SOMETIMES INFLUENCED 141 



of ancestral ids are still retained, side by side with tlie deter- 

 minants of the modem type.^ 



These aberrations, caused by the influence of temperature, are 

 in a certain degree hereditary. The experiments of Fischer on 

 Arctia Gaja, a species of butterfly with markedly vivid colouring, 

 have shown that a lowering of the temperature to — 8° C. during 

 the pupa stage will produce a dark variety, and that this variety 

 will be reproduced by the next generation, although in a much 

 lesser degree. This fact shows that the germ-plasm has been, 

 indeed, affected by the influence of the temperature along with 

 the soma, but not so strongly. The phenomenon is curious, for 

 it looks, at first sight, as if we had to deal with an inherited 

 somatic modification, whereas this is not the case. 



To sum up, we may say that, in the majority of cases, it is 

 very doubtful whether changes determined by climatic or 

 nutritive influences are hereditary ; the case of Nageli's 

 Hieracium, and of the galls produced on plants, show how great 

 may be the modifications effected in the soma, which, notwith- 

 standing, fail to affect the germ-plasm. Nevertheless, that such 

 environmental influences are capable occasionally of acting 

 directly on the germ-plasm is shown by the case of Vanessa. 

 And we may state, as a scientific possibihty, that climatic and 

 other environmental influences may alter the germ of a species, 

 so far as such alteration is compatible with the continued exist- 

 ence of that species. That is to say, if that species is physio- 

 logically incapable of adapting itself to environmental modifica- 

 tion, then that species must succumb. Selection is incapable of 

 adapting any species beyond certain limits of climatic or 

 nutritive change ; for there are temperatures above or below 

 which all life is impossible — at least, in the form in which we know 



^ Vide p. 79 with reference to the biogenetic law. We may note that 

 not all the determinants of the germ-plasm are modified as the result of 

 the lowering of the temperature, but only the determinants of the colouring 

 of the wings. This supphes another argument against the theory of a 

 homogeneous germ-plasm. 



