280 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
oy 
advantage of divergence of character,” nor ‘“ difference of external 
conditions,” nor all these taken together, nor any form of selection 
that may be hereafter discovered, is sufficient to account for diver- 
genee of character, but that another factor of equal if not superior 
importance must be recognized. In subsequent chapters I shall try to 
trace the causes on which this additional factor depends, and to indi- 
cate as far as possible the laws and relations under which they appear. 
DIVERGENT EVOLUTION NOT EXPLAINED BY NATURAL SELECTION. 
Natural selection is the exclusive generation of certain forms through 
the failure to live and propagate, of other kinds that are less adapted 
to the environment. 
In the case of the breeder, no selection avails anything that dces not 
result in some degree of exclusion. In the case of natural selection, 
where we are not considering ineffectual intentions, the selection is 
measured by the exclusion. Where there is no exclusion there is no 
selection, and where the exclusion is great the selection is severe. 
Moreover it is self-evident that there can be no crossing between the 
best fitted that survive and propagate and the least fitted that perish 
without propagating. To this extent, therefore, the prevention of 
crossing is complete. And further,it is evident that those whose 
meager fitness gives them but little opportunity for propagating will 
have a correspondingly diminished opportunity for crossing with the 
best fitted; and so on through the different grades of fitness, the power 
to affect the next genneration through having a share in propagating 
will measure the power to affect the progeny of the best fitted by cross- 
ing with them. It therefore follows that the freest crossing of the fit- 
test is with the fittest. 
Natural selection theretore proves to be a process in which the fittest 
are prevented from crossing with the less fitted through the exclusion of 
the iess fitted, in proportion to their lack of fitness. Through the pre- 
mature death of the least fitted, and the inferior propagation of the 
less fitted, there arises a continual prevention of crossing between the 
less fitted and the better fitted; and without this separation the trans- 
forming influence of the laws of organic life would have no power to 
operate. As Darwin has pointed out, the results produced by this 
removal of the less fitted and separate propagation of the better fitted 
closely correspond with those produced by the breeder, who kills off 
the less desirable individuals of his stock before they have an oppor- 
tunity to breed. The selection of the breeder avails nothing unless it 
leads to the determining of the kind that shall breed; and this he ean 
not accomplish without preventing free crossing with those that he 
does not desire. He must use some method to secure the separate 
breeding of the form that he desires to propagate. We therefore find 
in both natural and artificial selection the same fundamental method. 
