THE EVOLUTION OF LIVING BEINGS. 39 



,,One may be a mendelian, firmly believing in the 

 ,,principle of segregation following an F! generation, 

 which is the principal test of mendelism, and still 

 admit the probability of modifications from time to 

 ,,time of the stereochemistry of germ-plasma even in so 

 ,,called ,,pure lines". 



We may admit the possibility (because to deny a 

 possibility is unscientific) of such stereochemic modi- 

 fications, alias mutations, but there is no reason at all 

 to admit the probability ; all we can do, is to say that 

 the existence of such an inheritable changability of 

 the germplasma would be exceedingly interesting, but 

 that, so long as such proof is not forthcoming, we can 

 take no account of such a mere possibility in any effort 

 to explain evolution. 



Evolution has suffered quite sufficiently from the 

 ,,possibilities" with which it is charged, so that I fear 

 that the addition of another mere possibility might 

 be like the straw that broke the camel's back, e. g. 

 would throw the theory of evolution in universal dis- 

 credit. 



I hear it objected that the appearance of constant 

 new forms in small numbers from a certain form is good 

 evidence for mutation, because, if the new form were a 

 hybrid, it would have to segregate. This is of course not 

 true, if the original form is a heterozygote ; because 

 then the new form may be the product of the mating of 

 two identical gametes, produced by this heterozygote, 

 and consequently be homozygotic itself. 



Mutation can therefore, for the present, be discarded 

 as a factor in evolution ; what other kind of transmitta- 



