DARWIN'S THEORY 73 



conditions affecting the germ-cells and slightly affecting their 

 hereditary potentialities this is Darwin's account of the cause 

 of variation, especially of that kind of variation out of which 

 the differences between species have been built up ; and no 

 argument has yet been advanced to invalidate this view. It 

 must be remembered, however, that the experimental proof 

 of this view would require a period of time greatly exceeding 

 the span of a single human life, whereas the number of types 

 of germ-cells produced by a hybrid is a matter which can be 

 determined by experiments lasting only a few years. The 

 evidence in favour of Darwin's view must therefore for a long 

 time remain indirect ; but whoever will take the trouble to 

 compare the small wild horse of Northern Europe and Asia 

 pictures of which were scratched by primitive man on flint 

 and specimens of which can be seen in the London Zoological 

 Gardens, with the noble dray-horses which drag heavy loads 

 in the London streets, will form some conception of what may 

 be accomplished in thousands of years by steadily mating 

 together the strongest and best of a species. 



CHAPTER VII 



THE CONSEQUENCES OF DARWIN'S THEORY. THE 

 INTERPRETATION OF DEVELOPMENT. 



IN the preceding chapter we have given a brief sketch of 

 Darwin's theory of evolution and of the reasons for believing 

 it to be true. We have now to examine some of the conse- 

 quences which must follow if the theory be accepted. The 

 first of these consequences is that greater or less likeness 

 between two species of animals is prima facie evidence of 

 nearer or more distant blood relationship, and that the proper 

 classification of animals is one which most adequately expresses 

 this relationship. But animals in their growth from the germ 

 until they reach the adult condition pass through a great 

 number of stages in which they may present quite different 

 appearances, and the study of these stages is termed Embry- 

 ology. The most familiar example of this is found in the 

 life-history of the frog. As every one knows, the frog when 



