‘ 
DIVERGENT EVOLUTION THROUGH SEGREGATION. ad bal 
entirely fluctuating depends on some degree of positive segregation; 
therefore vigor and variation depend on the breaking down of incipient 
segregations, and on the interfusion of the slightly divergent forms 
that had been partially segregated. But in the history of every race 
that is winning success by its vigor and variation there is liable to 
come a time when some variety, inheriting sufficient vigor to sustain 
itself, even if limited to the benefits of crossing with the individuals of 
the same variety, becomes partially segregated. As we have already 
seen, segregation, in so far as it depends on the qualities of the organ- 
ism, tends ever to become more and more intense; but, in the very 
nature of things, not only will the segregation be for many generations 
only partial, but partial segregation, though it may greatly delay the 
submerging of different groups in one common group, will never pre- 
vent that result being finally reached. Though the siphon that con- 
nects two tanks of water be ever so small, the water will in time find a 
common level in both tanks, unless there are additions or subtractions 
of water that prevent such a result. So, in the case under considera- 
tion, final fusion will take place, unless differentiation progresses more 
rapidly than the fusion, or some other influence comes in to counteract 
the levelling influence of occasional crosses. If, under such conditions, 
some branch of the partially segregated variety becomes more fertile 
when generating with members of the same variety, and less fertile 
when generating with other varieties, a principle will be introduced 
tending to strengthen any form of partial segregation that already 
exists between the varieties. This principle when co-operating with 
partial segregation will produce pure masses of each variety, when, 
without the action of this principle, all distinctions would be absorbed 
by the crossing. We know that a transition from integrate fecundity 
to segregate fecundity usually takes place at a point in the history of 
evolution intermediate between the formation of an incipient variety 
and a strongly-marked species; and though the causes that produce 
this transition may be very difficult to trace, I believe the results that 
must follow can be pointed out with considerable clearness and cer- 
tainty. 
Darwin’s investigations have shown that in many cases, if not in the 
majority, the relation of varieties to each other is that which I have 
‘alled integrate fecundity and integrate vigor; that is, the highest 
fertility is attained when varieties are crossed, and the vigor of off- 
spring thus produced is greater than when the inter-generation is within 
the limits of one variety. He, however, gives in ‘ Variation under 
domestication,” chapter xvi, some special cases, in which ‘varieties 
of the same species behave, when crossed, like closely allied but distinet 
species ;” and remarks that similar cases “‘may not be of very rare oc- 
currence, for the subject has not been attended to.” The same cases 
are also mentioned in all the editions of the “Origin of Species.” * 
* See Ist edition, p. 238; 5th edition, p. 259: 6th edition, p. 258. 
