Distinct genera are fertile inter se. 
“The hybrids from the common and Chinese geese (A. cygnoides) 
—species which are so different that they are generally ranked in 
distinct ge?tera—have often bred in this country with either pure 
parent, and, in one single instance, they have bred inter se. This 
was effected by Mr. Eyton, who raised two hybrids from the same 
parents, but from different hatches ; and from these two birds he 
raised no less than eight hybrids (grandchildren of the pure geese) 
from one nest. In India, however, these cross-bred geese must be 
far more fertile ; for I am assured by two eminently capable judges— 
namely, Mr. Blyth and Captain Hutton—that whole flocks of these 
crossed geese are kept in various parts of the country ; and as they 
are kept for profit, where neither pure parent-species exists, they 
must certainly be highly or perfectly fertile.”—(Origin of Species. 
P- 24.0.) 
“ Dean Herbert’s experiments are most instructive in this respect, 
since they show that in a considerable number of large genera, 
hybrids are perfectly fertile, and not unfrequently more fertile than 
the parents, while in many cases they produce quite fertile offspring ; 
and he concludes ‘ that the sterility or fertility of the offspring does 
not depend upon original diversity of stock ; and that, if two species 
are to be united in a scientific arrangement on account of a fertile 
issue, the botanist must give up his specific distinctions generally, 
and entrench himself within genera.’ He showed that many very 
distinct species of crinum, hippeastrum, gladiolus, pelargonium, 
calceolaria, and many other genera were quite fertile when crossed, 
and often produced offspring which could be propagated indefinitely, 
and have thus formed valuable garden flowers ; while other species, 
more alike externally, either could not be crossed at all, or produced 
offspring which were sterile; and he thence concludes ‘ that the 
fertility of the hybrid or mixed offspring depends more upon the 
constitutional than the closer botanical affinities of the parents.’ ” 
—(Wallace. Fortnightly Review, vol. xl., N.S., p. jii.) 
Distinct families are fertile inter se. 
“ The mere fact that not only animals of distinct genera, but even 
those classed in distinct families—as the pheasant and black 
grouse—sometimes produce hybrid offspring in a state of nature, 
is itself an argument against there being any constant infertility 
between the most closely allied species, since if that were the case 
we should expect the infertility to increase steadily with remoteness 
of descent till, when we came to family distinctions, absolute sterility 
should be invariable.”—(Wallace. Fortnightly Review, vol. xl., 
N.S., p. 311.)