196 MR. C. DARWIN ON LYTHRUM SALICARIA. 
some of the lower animals males, females, and hermaphrodites 
of the same species; we have the somewhat more curious case 
of certain Cirripedes which are hermaphrodites, but are sexually 
aided by whole clusters of what I have called complemental 
males; we have, as Mr. Wallace has lately shown, the females of 
certain Lepidoptera existing under three distinct forms; but in 
none of these cases is there any reason to suspect that there 
is more than one female or one male sexual element. With 
certain insects, as with Ants, in which there exist, besides 
males and females, two or three castes of workers, we have a 
slightly nearer approach to our case, for the workers are so far 
sexually affected as to have been rendered sterile. With plants, 
at least with phanerogamic plants, we have not that wonderful 
series of successive developmental forms so common with animals ; 
nor could this be expected, as plants are fixed to one spot from 
their birth, and must be adapted throughout life to the same 
conditions. With plants we have sexual differences in structure, 
but apparently less strongly marked than with animals, from 
causes which are in part intelligible, such as there being no 
sexual selection; again, we have that class of dimorphic flowers 
so ably discussed recently by Hugo von Mohl, in which some 
of the flowers are minute, imperfectly developed, and neces- 
sarily self-fertile, whilst others are perfect and capable of 
crossing with other flowers of the same species; but in these 
several cases we have no reason to suspect that there is more 
than one female or one male sexual element. When we come to 
the class of reciprocally dimorphic plants, such as Primula, Linum, 
&e., we first meet with two masculine and two feminine sexes. 
But these cases, which seemed only a short time since so strange, 
now sink almost into insignificance before that of the trimorphie 
species of Zythrum. 
Naturalists are so much aceustomed to behold great diversities 
of structure associated with the two sexes, that they feel no 
surprise at the fact; but differences in sexual nature have been 
thought to be the very touchstone of specific distinction. We 
now see that such sexual differences—the greater or less power 
of fertilizing and being fertilized—may characterize and keep 
separate the coexisting individuals of the same species, in the 
same manner as they characterize and have kept separate those 
groups of individuals, produced from common parents during the 
lapse of ages or in different regions, which we rank and deno- 
minate as distinct species. 
