EVOLUTION 



1049 



Lamarckian answer involves the postulate of the transmissibility 

 of modifications, especially of functional modifications; and the 

 evidence of this, conveniently summed up by Kammerer (1924) 

 and by MacBride (1924), seems to us still inconclusive. The inter- 

 pretation which seems to involve fewest assumptions is that sug- 

 gested by Darwin. In discussing acclimatisation (1868), Darwin 

 laid most emphasis on the natural selection of spontaneous varia- 

 tions. As to these variations he expressly says "there is no evidence 

 that a change in the constitution of the offspring necessarily stands 

 in any direct relation with the nature of the climate inhabited by 

 the parents". In regard to selection, he lays emphasis on two 

 points, (a) the organism's power of resistance to difficult conditions 



Fig. 180. 



Two Variants (A and B) of the Potato Beetle {Leptinotarsa decemlineata) . 



After Tower. 



in the new climate, and {h) some useful change in the period of 

 reproduction, such as earlier flowering and fruiting. 



To Darwin's interpretation an addition may be suggested. It is 

 conceivable that the climatic peculiarities may affect the meta- 

 bolism of the organism through and through, and may thus serve 

 as stimuli to the variability of the germ-cells. If the climatic pecu- 

 liarity should induce [a) an adaptive modification in the body of 

 the organism and [h), at the same time, a variation in the germ- 

 cells which finds expression as a similar new character in the off- 

 spring, the phenomenon is called "parallel induction". It must be 

 distinguished theoretically from the transmission of a somatic 

 modification ; it is, as Weismann said, a change induced in the germ- 

 cells along with, but not through, the bodily modification. 



But it is possible that climatic peculiarities may penetrate into 

 the germ-cells and affect them without producing any modification 

 in the body. Thus W. L. Tower subjected full-grown potato-beetles 

 (Leptinotarsa) to peculiar conditions of temperature and humidity 

 during the time when the eggs were maturing, and found that 



