1050 LIFE : OUTLINES OF GENERAL BIOLOGY 



"mutations" occurred in a certain proportion of the offspring. The 

 parents were not affected, having passed the plastic stage ; and some 

 of the eggs were not affected at all. Moreover, the same environ- 

 mental peculiarity, analogous to a climatic change, did not always 

 evoke the same mutation in the offspring. Some of the mutations 

 in colour and markings were very striking; others affected minute 

 details of structure. There was no reversion to the parental condi- 

 tion. This case is of particular interest in connection with acclima- 

 tisation; for the artificial environmental conditions, effected in 

 large steel and glass cages, were comparable to different climatic 

 conditions in which different species of potato-beetle live. 



By Muller in particular it has been shown that an exposure of 

 the germ-ceUs of the fruit-fly (Drosophila melanogaster) to appro- 

 priate doses of X-rays, is followed by numerous non-fatal mutations, 

 which often breed true. Now there is a variable amount of gamma 

 radiation in natural conditions, and it is possible that the amount 

 present may be sufficient to induce mutations comparable to those 

 which can be induced experimentally. In any case, the experi- 

 mental irradiation of the ova and sperms of Drosophila illustrates 

 the possible action of the environment as a variational stimulus. 

 A mutation following a climatic change might have this origin. 

 As Weismann said many years ago (1904, p. 269): 'Tt does not 

 seem impossible that the climate may have a variational influence 

 upon certain determinants of the germ-plasm, for we have already 

 seen that the influence of cultivation may incite plants and animals 

 to hereditary variations, and that slowly increasing disturbances in 

 the equilibrium of the determinant system may thereby be produced, 

 which may suddenly find marked expression as 'mutations'. But 

 there is little probability that adaptations, that is, transformations 

 corresponding to the altered climate, can arise in this way!" The 

 meaning of this last sentence is that Weismann believed these 

 adaptations were the outcome of the prolonged natural selection of 

 fluctuating variations in the germ-plasm. 



If the thick coat of the Tibetan Long-nosed Monkey (Rhino- 

 pithecus), which seems to be so indifferent to the snow, is a heredi- 

 tary character, as it seems to be, and not an individual modification, 

 re-imposed on each successive generation, there arises the usual 

 objection to the Darwinian theory that there is little likelihood of 

 such a suitable spontaneous variation arising. This difficulty, which 

 Darwin frankly faced, will be lessened if it can be shown experi- 

 mentally that such a factor as cold tends to arouse a restricted 

 number of mutations. And, apart from this, it must be kept in mind 

 that those mutations are, on general grounds, most likely to arise 

 and persist which are congruent with the germinal organisation 

 that has been already established. The impression of fortuitousness 

 is further lessened when we keep in mind the likelihood of correlated 



