OF CHLOËON (EPHEMERA) DIMIDIATUM. 489 
fast, and consequently feeding voraciously. Nor, even if the change could be thus 
effected, would the mouth, in its intermediate stages, be in any way fitted for biting and 
chewing leaves. The same reasoning applies also to the digestive organs. Hence the 
caterpillar undergoes little, if any, change, except in size, and the metamorphosis is con- 
centrated, so to say, into the last two moults. The changes then become so rapid and 
extensive, that the intermediate period is necessarily one of quiescence. 
Owing to the fact that the organs connected with the reproduction of the species come 
to maturity at a late period, larvæ are generally incapable of breeding. There are, how- 
ever, some flies which have viviparous larvæ, and thus offer a typical case of alternation 
of generations, owing to the early period of leaving the egg, and the action in many cases 
of external cireumstances on the larva different from those which affect the mature form. 
Thus, then, we find among insects every gradation, from the case of simple growth to 
that of alternation of generations; and we see how from the single fact of the early period 
at which certain animals quit the egg, we can account for the metamorphoses they 
go through, and the still more remarkable phenomenon that, among many of the lower 
animals, the species is represented by two very different forms. We may even, from the 
same considerations, see reason to conclude that this phenomenon may in the course of 
ages become still more common than it is at present. As long, however, as the external 
organs arrive at their mature form before the internal generative organs are fully deve- 
loped, we have cases of metamorphosis; but if the reverse is the case, then alternation 
of generations often results. i ; 
The same considerations throw much light on the remarkable fact, that in alternation 
of generations the reproduction is always agamic in the one form. This results from 
the fact that impregnation requires the perfection both of the external and internal 
organs; and if the phenomenon arises, as has just been suggested, from the fact that = 
internal organs arrive at maturity before the external ones, impregnation cannot take 
place, and reproduction will result in those species only which have the power of agamic 
multiplication. 
Moreover it is evident that we have in th 
The term has usually been applied to those case 
themselves at maturity se ak different forms. The different forms of Ants and mes 
eit à : o plants the remarkable case of 
afford us familiar instances among animals; and among p visui tend 
the genus Primula has recently been worked out with his usual wir y my sem 
Mr.Darwin. Even more recently he has made known to us the =. cst M 
phenomenon afforded by the genus Linum, in which there are three distinc , 
Whieh therefore offers an instance of polymorpltism*. tom the fnt in resulting 
The other kind of dimorphism or polymorp — t on the mature, but on the 
from the differentiating action of external circumstances, not 0 iai 
x wards one another in a re- 
Young individual. The different forms, therefore, stand to 
' : - divides at the extremity; in 
lation of succession. In the first case the chain veni of dimorphism under this 
the other it is composed of dissimilar uto of alternation of generations. 
second form have been described under the n : à 
n which the sexes are distinct are truly imorphic. 
e animal kingdom two kinds of dimorphism. 
s in which animals or plants present 
* Indeed all animals i 
