Centennial of Charles Darwin 375 



structure and form, in the one case, and as to the ability 

 for individuals belonging to any group of organized be- 

 ings to intermarry and produce a fertile offspring, in 

 the other. Both considerations are to be kept in view in 

 determining what is to be regarded as a species, but 

 especially the latter point. It is found, for instance, that 

 the male mule, which is a cross between the horse and the 

 ass, cannot give birth to another mule. Plants, in like 

 manner, may be crossed, but the hybrids thus produced 

 are infertile. This is one of the mysteries of the realm 

 of nature. How organized beings came to be thus 

 divided up into groups, and surrounded by a wall of 

 separation, was the problem Avhich Darwin set himself to 

 solve. The task he placed before himself was this : to 

 show that all the various forms of vegetable and animal 

 life with which the globe is now peopled, or of which we 

 find the remains preserved in a fossil state in the great 

 earth museum around us, have come down from at least 

 four or five progenitors, animals and plants in an equal 

 number. But his speculation did not stop at that point 

 and he adds, "Analogy would lead me one step further, 

 namely, to the belief that all animals and plants have 

 descended from some one prototype and that probably 

 all the organic beings which have ever lived on this 

 earth have descended from some one primordial form 

 into which life was first breathed by the Creator." 



The main propositions by which he would bring us 

 to that conclusion may be summed up, as follows: 



(1) That observed and admitted variations spring- 

 up in the course of descent from a common progenitor. 

 (2) That many of these variations tend to an improve- 

 ment upon the parent stock, possessing some quality that 

 is profitable or advantageous. (3) That by a continued 

 selection of these improved specimens, as the progenitors 

 of future stock, its powers may be increased inimitably. 

 (4) That there is in nature a power continually and 

 universally working out this selection and so fixing and 



