THE KINSHIP OF LIFE. 15 
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- 
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 
Darwin’s 
answer. 
