Definite Variation 99 
fertile on the female side, and some on the male side also, but the 
hybrids produced between the Turnip (Brassica napus) and the 
Swede (Brassica campestris), which, according to our estimates of 
affinity, should be nearly allied forms, are totally sterile’. Lastly, it 
may be recalled that in sterility we are almost certainly considering a 
meristic phenomenon. Failure to divide is, we may feel fairly sure, 
the immediate “cause” of the sterility. Now, though we know very 
little about the heredity of meristic differences, all that we do know 
points to the conclusion that the less-divided is dominant to the 
more-divided, and we are thus justified in supposing that there are 
factors which can arrest or prevent cell-division. My conjecture 
therefore is that in the case of sterility of cross-breds we see the 
effect produced by a complementary pair of such factors. This and 
many similar problems are now open to our analysis. 
The question is sometimes asked, Do the new lights on Variation 
and Heredity make the process of Evolution easier to understand ? 
On the whole the answer may be given that they do. There is some 
appearance of loss of simplicity, but the gain is real. As was said 
above, the time is not ripe for the discussion of the origin of species. 
With faith in Evolution unshaken—if indeed the word faith can be 
used in application to that which is certain—we look on the manner 
and causation of adapted differentiation as still wholly mysterious. 
As Samuel Butler so truly said: “To me it seems that the ‘Origin of 
Variation,’ whatever it is, is the only true ‘Origin of Species’”?, and 
of that Origin not one of us knows anything. But given Variation— 
and it is given: assuming further that the variations are not guided 
into paths of adaptation—and both to the Darwinian and to the 
modern school this hypothesis appears to be sound if unproven—an 
evolution of species proceeding by definite steps is more, rather than 
less, easy to imagine than an evolution proceeding by the accumulation 
of indefinite and insensible steps. Those who have lost themselves in 
contemplating the miracles of Adaptation (whether real or spurious) 
have not unnaturally fixed their hopes rather on the indefinite than 
on the definite changes. The reasons are obvious. By suggesting 
that the steps through which an adaptative mechanism arose were 
indefinite and insensible, all further trouble is spared. While it 
could be said that species arise by an insensible and imperceptible 
process of variation, there was clearly no use in tiring ourselves by 
trying to perceive that process. This labour-saving counsel found 
great favour. All that had to be done to develop evolution-theory 
was to discover the good in everything, a task which, in the complete 
absence of any control or test whereby to check the truth of the 
1 See Sutton, A. W., Journ. Linn. Soc. xxxvit. p. 341, 1908. 
2 Life and Habit, London, p. 263, 1878. 
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