xT EVOLUTION IN BIOLOGY 223 
valid explanation of the occurrence of varieties and 
races ; and they saw clearly that, if the explanation 
would apply to species, it would not only solve the 
problem of their evolution, but that it would ac- 
count for the facts of teleology, as well as for those 
of morphology ; and for the persistence of some 
forms of life unchanged through long epochs of 
time, while others undergo comparatively rapid 
metamorphosis. 
How far “ natural selection” suffices for the pro- 
duction of species remains to be seen. Few can 
doubt that, if not the whole cause, it is a very 1m- 
portant factor in that operation ; and that it must 
play a great part in the sorting out of varieties 
into those which are transitory and those which 
are permanent. 
But the causes and conditions of variation have 
_ yet to be thoroughly explored ; and the importance 
of natural selection will not be impaired, even if 
further inquiries should prove that variability 
is definite, and is‘determined in certain-directions 
rather than in others, by conditions inherent in 
that which varies. It is quite conceivable that 
every species tends to produce varieties of a 
limited number and kind, and that the effect of 
natural selection is to favour the development of 
some of these, while it opposes the development 
of others along their predetermined lines of modi- 
fication. 
7. No truths brought to light by biological 
