154 The American Naturalist. [September, 
and the opportunities or occasions of procreation multiplied. 
Again it is necessary to consider a reversal of this. The 
sterility of the offspring of crossed parents of different species 
points to the fact that there are or may be functional changes 
in the powers of generation, and that the new species, is, by this 
law, made dependent for its successful extension, upon the 
intercourse of similar individuals. It is likely that in connect- 
ion with the rise of a new species those organs concerned in 
reproduction have become modified, and the system of seminal 
secretion, which carries with it the power of perpetuating the 
new forms, has itself been more or less profoundly affected. 
From such considerations it seems fairly probable that new 
species appear in limited numbers, and acquire after time the 
full power of propagation until with increasing numbers the 
maximum of their numerical rise is reached, and then that 
decadence begins which ends in their disappearance. It will 
be understood that by “limited numbers” we mean such re- 
presentations of species as are much below their later and 
more normal development. 
It then appears from such considerations, without further 
detail, that the factors of numerical increase are two, the exter- 
nal or physical conditions of life, and the internal or biogenet- 
ic force of propagation. As regards the first, the external or 
physical conditions of life, it may be assumed that the appear- 
ance of a species must take place under favorable conditions, 
if we are to accept the Darwinian hypothesis, that specific 
origination means that very thing, the better adaptation of new 
species to reigning conditions than any other, for it is its pre- 
ponderant aptitude for life under these conditions that brings 
the new species into existence. So that as regards the encour- 
agement to increase given by the external conditions it is un- 
exceptional or adequate, and the rate of multiplication is then 
made dependent upon the physiological factor, the power and 
provision for propagation. These favorable conditions will 
be temporary. They will be succeeded by others less favor- 
able, and the species, started under way under the best exter- 
nal auspices will begin to work against physical detriments 
and brakes that will lower its vital momentum, and, unless 
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