THE PROBLEM OF SPECIES 5 
kind of work have set themselves for solution is that 
of the nature and method of origin of the existing 
differences between certain groups of organic beings 
—namely, species. Basing their studies on the doc- 
trine that the present species have arisen through the 
modification of pre-existing species, they endeavour 
to observe how modifications of existing species do 
actually arise in Nature, as well as under domestica- 
tion ; and they watch the hereditary transmission of 
the modified forms when like is bred with like, and 
when different types are crossed together. For the 
theory of uniformity, now universally accepted, teaches 
us that the organisms with which we are now familiar 
owe their present characteristics to the accumulation 
of a series of changes similar to those which are still 
in progress. It has, therefore, appeared likely to a 
few that a further understanding of the processes of 
evolution might best be obtained by a closer study, 
firstly, of variation, or the ways in which offspring 
differ from their parents ; and, secondly, of inheritance, 
or the ways in which the resemblances between parents 
and their offspring are perpetuated from one genera- 
tion to another. 
It may be well to point out at once that the further 
study of the method of origin of new species, admitting, 
as it does, that this process is not yet by any means 
fully understood, does not for this reason imply that 
the theory of organic evolution itself is open to criti- 
cism. The evidence that new species arise by the 
modification of pre-existing species is quite indepen- 
dent of the evidence that this process invariably occurs 
