5 8z THE POPULAR SCIENCE MONTHLY. 



an explanation began to seem very improbable, and it was com- 

 pletely discredited by the fact that many kinds of plants and ani- 

 mals have persisted with little or no change during several suc- 

 cessive periods, side by side with other kinds in which there has 

 been extensive variation and extinction. It was further observed 

 that between the forms of successive periods in the same geo- 

 graphical regions there was a manifest family likeness, indicating 

 that the later were connected with the earlier through the ordinary 

 bonds of physical descent. A host of facts from comparative mor- 

 phology and embryology went to confirm this inference ; and so, 

 when after nearly twenty years of incubation Mr. Darwin was 

 ready to plant the seeds of his remarkable theory, he found the soil 

 very thoroughly prepared and fertilized in which to plant them. 

 All that men were waiting for was the discovery of a vera causa, 

 All that was wanted was to be able to point to some one agency, 

 similar to agencies now in operation and therefore intelligible, 

 which could be proved to be capable of making specific changes in 

 plants and animals. Mr. Darwin's solution of the problem was so 

 beautiful, it has become so generally accepted and so deeply inter- 

 fused into all the thinking of our time, it seems now so natural 

 and so inevitable, that we may be in danger of forgetting that the 

 problem was really one of the most complicated and abstruse that 

 the scientific mind has ever grappled with. Starting from the 

 known experiences of breeders of domestic animals and cultivated 

 plants, and duly considering the remarkable and sometimes won- 

 derful changes that are wrought by the simple process of selection, 

 the problem before Mr. Darwin was to detect among the multifa- 

 rious phenomena of organic nature any agency capable of accom- 

 plishing what man thus accomplishes by selection. In detecting 

 the agency of natural selection, working perpetually through the 

 preservation of favored individuals and races in the struggle for 

 existence, Mr. Darwin found the vera causa for which men were 

 waiting. With infinite patience and caution he applied his meth- 

 od of explanation to one group of organic phenomena after an- 

 other, meeting in every quarter with fresh and often unexpected 

 verification. He had the satisfaction of living to see pretty much 

 the whole contemporary world of zoologists, botanists, and palae- 

 ontologists pursuing the lines of investigation which he had laid 

 down and in general agreement as to the fundamental principle. 

 There was a general acquiescence in natural selection as an 

 agency capable of working specific changes, while further specu- 

 lation and investigation in all directions were employed in ascer- 

 taining the precise character of its work and determining the lim- 

 its of its efficacy. That all the phenomena of the organic world 

 can be accounted for by natural selection, Mr. Darwin never at 

 any time supposed ; nor was he ever so silly as to suppose that all 



