THE KINSHIP OF LIFE. 3 
plasm, which is, so far as we know, the physical basis of 
all life; and the equally mysterious nuclear substance 
or chromatin which in some fashion presides over all 
the movements of the protoplasm and is the physical 
basis of the phenomena of heredity. The same laws of 
heredity, variability, and of response to outside stimulus 
hold in all parts of the organic world. All organisms 
have the same need of reproduction. All are forced to 
make concession after concession to their surroundings, 
and in these concessions all progress in life consists. 
And at last each organism or each alliance of organisms 
must come to the greatest concession of all, which we 
call death. 
The unity in life is then not less a fact than is life’s 
great diversity. Whatever the emphasis we may lay 
upon the diversity of life, the essential 
unity of all organisms must not be for- 
gotten. This fundamental likeness among widely varied 
forms stands as the basis of all classification. It is this 
only which makes classification possible or conceivable. 
These bonds of union, which are real as distinguished 
from resemblances which are merely superficial or ap- 
parent, are known to the naturalist as homology. The 
existence of homologies is the fundamental fact in bio- 
logical science. It has been regarded as a mystery of 
mysteries, but this mystery assumes the form of natural 
law in the light of the plain fact that identity of structure 
is the simple result of identity of parentage. Homology 
in any form is simply the stamp of heredity. In other 
words, homology means blood-relationship. The sim- 
plest explanation is the truest and would long ago have 
been recognized had it not been for prejudices of va- 
rious sorts—theological prejudices that saw the image 
of God in man only, and scientific prejudices which 
arose from the surface study of surfaces. For it is the 
Unity in variety. 
