WHAT IS EVOLUTION? 31 



last tenet of heredity has, however, of late been greatly 

 shaken by the investigations of Weismann, which 

 have thrown doubt on the possibility of inheritance of 

 some characters acquired by the individual. We shall 

 see that if these new views are established, the whole 

 aspect of the question of specific modification will be 

 greatly changed. Since, however, no case establish 

 ing any one of the alleged factors of new species is 

 actually known to have occurred, these doctrines of 

 modification and heredity, as applied to the origin of 

 species, are, as yet, articles of faith and not of scientific 

 certainty, and the whole question of causation in evo 

 lution may be said to be in an uncertain and transi 

 tion state. 



In these circumstances the questions as to possible 

 modes of development may seem to lose much of 

 their importance ; but the disciples of Darwin inform 

 us that, independently of known and ascertained 

 causes, the probability of development which arises 

 from embryonic analogy and the affinities of animals 

 and plants among themselves, is so great that the 

 doctrine must nevertheless be credited or at least 

 treated with respect. Farther, the modes of develop 

 ment are, as we have akeady seen, the only points on 

 which certain evidence can be obtained. It is neces 

 sary, therefore, to consider these. 



Here we must admit, in the first place, that 

 though we can study modes of variation of species, 

 no case has actually occurred under the observation! 



