| Sune 19, 1873] 
a a 
ee ee 
the usual character, and eventually become gnats. His 
observations have been confirmed, as regards this main 
fact, by other naturalists ; and there can, I think, be no 
doubt that they are, in the main, correct. 
Here, then, we have a distinct case of alternation of 
generations, as characterised by. Steenstrup. Probably 
other cases will be discovered in which insects undeniably 
in the larval state will be found to be fertile. Nay, it 
seems to me possible, if not probable, that some larvz 
which do not now breed, in the course of ages may come 
to do so. 
If this idea is correct, it shows how the remarkable 
phenomenon known as alternation of generations may 
have originated. At any rate, we find among insects every 
mode of development ; from simple growth on the one 
hand, to well-marked alternation on the other. In the 
wingless species of Orthoptera there is little difference, 
excepting in size, between the young larva and the 
perfect insect. The growth is as simple and gradual as 
in any other animal; and the creature goes through 
nothing which would, in ordinary language, be called a 
metamorphosis. In the majority of Orthoptera the pre- 
sence of wings produces a marked difference between the 
larva and the imago. The habits, however, are nearly 
the same throughout life, and consequently the action of 
external circumstances affects the larva in the same 
manner as the perfect insect. 
This is not the case with the Ephemeride. The larve 
do not live under the same conditions as the perfect 
insects; external forces accordingly affect them in a 
different manner; and we have seen that they pass 
through some changes which bear no reference to the 
form of the perfect insect: these changes, however, are 
for the most part very gradual. The caterpillars of 
Lepidoptera have even more extensive changes to under- 
go; the mouth of the larva, for instance, is remarkably 
unlike that of the perfect insect. A change in this 
organ, however, could hardly take place while the insect 
was still growing fast, and consequently feeding voraci- 
ously. 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 concentrated, 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, larvee are generally incapable of breeding. There 
are, however, some flies which have viviparous larve, 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 circumstances 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 their metamorphoses and for 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 considera- 
tions, 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 developed, we have cases of metamor- 
phosis ; but if the reverse is the case, then alternation of 
generations often results. 
The same considerations throw much light on the 
remarkable fact, that in alternation of generations the re- 
production is, as a general rule, agamic in the one form. 
NATURE 
145 
This results from the fact that reproduction by distinct 
sexes requires the perfection both of the external and 
internal organs; and if the phenomenon arise, as has 
just been suggested, from the fact that the internal 
organs arrive at maturity before the external ones, re- 
production will result in those species only which have 
the power of agamic multiplication. 
Moreover it is evident that we have in the animal 
kingdom two kinds of dimorphism. 
This term has usually been applied to those cases in 
which animals or plants present themselves at maturity 
under two different forms. The different forms of ants 
and bees afford us familiar instances among animals ; 
and among plants the remarkable case of the genus 
Primula has recently been worked out with his usual 
ability by Mr. Darwin. Even more recently he has made 
known to us the still more remarkable phenomenon 
afforded by the genus Zzwm, in which there are three 
distinct forms, and which therefore offers an instance of 
polymorphism.* 
The other kind of dimorphism or polymorphism differs 
from the first in resulting from the differentiating action 
of external circumstances, not on the mature, but on the 
young individual. The different forms, therefore, stand 
towards one another in a relation of succession. In the 
first case the chain of being divides at the extremity ; in 
the other it is composed of dissimilar links. Many cases 
of dimorphism under this second form have been de- 
scribed under the name of alternation of generations, 
The term, however, has met with much opposition, and 
is clearly inapplicable to the differences exhibited by 
insects in different periods of their life. Strictly speaking 
the phenomena are very frequently not alternate, and, in 
the opinion of many eminent naturalists, they are not 
cases of generation at all. ¢ 
In order, then, to have some name for these remarkable 
phenomena, and to distinguish them from those cases 
in which the mature animal or plant is represented by 
two or more different forms, I think it would be con- 
venient to retain for these latter exclusively the terms 
dimorphism and polymorphism; and those cases in 
which animals or plants pass through a succession of 
different forms might be distinguished by the name of 
dieidism or polyeidism. 
The conclusions, then, which I think we may draw 
from the preceding and other considerations are :— 
1. That the occurrence of metamorphoses arises from 
the immaturity of the condition in which some animals 
quit the egg. 
2. That the form of the insect larva whenever it 
departs from the original vermiform type, depends in 
great measure on the conditions in which it lives. The 
external forces acting upon it are different from those 
which affect the mature form; and thus changes are 
produced in the young which have reference to its imme- 
diate wants, rather than to its final form. 
3, That metamorphoses may therefore be divided into 
two kinds, developmental and adaptational. . 
4. The apparent abruptness of the changes which in- 
sects undergo arises in great measure from the hard- 
ness of their skin, which admits no gradual alteration of 
form, and which is itself necessary in order to afford suffi- 
cient support to the muscles. 
5. The immobility of the pupa or chrysalis depends on 
the rapidity of the changes going on in it. 
6. Although the majority of insects go through three 
well-marked stages after leaving the egg, still a large 
number arrive at maturity through a somewhat indefinite 
number of slight changes. 
* Of course all animals in which the sexes are distinct are in one sense 
dimorphic. S = 
+ ‘There is no such thing asa true case of ‘alternation of generations 
in the animal kingdom ; there is only an alternation of true generation with 
the totally distinct process of gemmation or fission.”—Huxley on Animal 
Individuality, Ann. and Mag. of Nat. Hist., June 1852. 
