96 
ON THE ADVANTAGE 
Chap. IV. 
beings, or of any great and sudden modification in tlieir 
structure. 
On the Intercrossing of Individuals ,—I must here 
introduce a short digression. In the case of animals 
and plants with separated sexes, it is of course obvious 
that two individuals must always (with the exception of 
the curious and not well-understood cases of partheno¬ 
genesis) unite for each birth; but in the case of her¬ 
maphrodites this is far from obvious. Nevertheless I am 
strongly inclined to believe that with all hermaphrodites 
two individuals, either occasionally or habitually, concur 
for the reproduction of their kind. This view was first 
suggested by Andrew Knight. We shall presently see 
its importance; but I must here treat the subject with 
extreme brevity, though I have the materials prepared 
for an ample discussion. All vertebrate animals, all 
insects, and some other large groups of animals, pair 
for each birth. Modern research has much diminished 
the number of supposed hermaphrodites, and of real 
hermaphrodites a large number pair; that is, two indivi¬ 
duals regularly unite for reproduction, which is all that 
concerns us. But still there are many hermaphrodite 
animals which certainly do not habitually pair, and a 
vast majority of plants are hermaphrodites. What 
reason, it may be asked, is there for supposing in these 
cases that two individuals ever concur in reproduction ? 
As it is impossible here to enter on details, I must trust 
to some general considerations alone. 
In the first place, I have, collected so large a body of 
facts, showing, in accordance with the almost universal 
belief of breeders, that with animals and plants a cross 
between different varieties, or between individuals of 
the same variety but of another strain, gives vigour and 
