To prove that all birth variations are not those neces 
sarily and inevitably associated with sexual reproduction, 
we may first adduce the fact that the members of different 
species unite to produce offspring which more or less 
partakes of the likeness of both parents. “ If,” says 
M. Flourens, “ species could change, hybridisation would 
assuredly be the most direct and efficacious means of 
bringing about this change.* . . . If the hybrids 
could perpetuate themselves indefinitely, hybrids would 
form species, as many new species as there were 
hybrids.”-j- But he contends that “hybridisation is not 
in any case nor in any sense, neither for vegetables 
or animals, a source of new species.” \ Three arguments 
are adduced in favour of this assertion, (i) It is said that 
individuals of the same species and varieties of the same 
species, are perfectly fertile inter se. (2) It is contended 
that the offspring of different species have a limited fertility. 
(3) It is maintained that the offspring of hybrids are either 
infertile or lapse into one or other of the parental forms. 
Now, we may grant the truth of these assertions, so 
far as to admit that these phenomena do sometimes 
take place. But it does not therefore follow that there 
is always this fertility between individuals of the same 
species and the varieties of the same species ; that there 
is always a limited fertility between the offspring of differ 
ent species ; or that the continuous fertility of hybrids 
which maintain their hybrid organisation is always im 
possible in nature. 
Neither individuals nor varieties of the same species are 
always fertile inter se. 
“ It is impossible to resist the evidence of the existence of a 
certain amount of sterility (in the varieties of the same species; 
* Exatnen du Livve de M. Darwin sur I'Origine des Espices. p. 9/. 
t Ibid. p. g2. i p. 117.