THE KINSHIP OF LIFE. ! 5 



continents are modified descendants of extinct forms? 

 But if this be so, what certainty have we that other 

 creatures have not been similarly modified ? And may 

 they not be still undergoing modification ? Then why 

 may not the origin of species be due to descent with 

 modifications ? The difference in species would then be 

 the result of the influences which make for change, and 

 the unity would be due simply to the action of the law 

 of the heredity. 



And this is the theory which Darwin finally reached. 

 The unity would be accounted for easily enough, for by 

 this view homology is the simple index of common he- 

 redity. The fact of variation could be shown, but what 

 could be the cause of variations so universal and on 

 such a grand scale as we find them in Nature ? If this 

 law could be worked out, then the innumerable facts 

 of homology and variation would have a meaning in- 

 stead of being as before so many isolated curiosities of 

 Nature. To the working out of this law he gave 

 twenty-five years of his life, gathering information from 

 every source accessible to man. 



To the famous botanist, Joseph D. Hooker, Darwin 



wrote in 1844: "Besides a general interest about the 



southern lands, I have been now ever 



since my return engaged in a very pre- 

 answer. ' * * 



sumptuous work, and I know no one 



individual who would not say a very foolish one. I was 

 so struck with the distribution of the Galapagos organ- 

 isms and with the character of the American fossil mam- 

 mifers that I determined to collect blindly every sort of 

 fact which could bear in any way on what are species. 

 I have read heaps of agricultural and horticultural books 

 and have never ceased collecting facts. At last gleams 

 of light have come, and I am almost convinced (quite 

 contrary to the opinion I started with) that species are 

 3 



