488 SIR J. LUBBOCK ON THE DEVELOPMENT 
phenomenon which is known as the ** Alternation of Generations," for the first sys- 
tematic view of which we are indebted to my eminent friend Prof. Steenstrup. 
I have always felt it very difficult to understand why any species should have been 
created in this double character; nor, so far as I am aware, has any explanation of the 
fact yet been attempted. Yet insects offer, in the metamorphoses which they go through, 
a phenomenon not altogéther dissimilar, and give a clue to the manner in which alter. 
nations of generations may have originated. 
The Caterpillar owes its difference from the Butterfly to the early stage at which it 
leaves the egg; but its actual form is mainly due to the influence of the conditions in 
which it lives. If the Caterpillar, instead of changing into one Butterfly, produced 
several Butterflies, we should have an instance of alternation of generations. Until lately, 
however, we knew of no such case; each larva produced one imago, and that not by 
generation but by development. It has long been known, indeed, that there are some 
species in which certain individuals remain always apterous, while others acquire wings 
Many entomologists, however, regard these abnormal individuals as perfect though 
wingless insects; and therefore, though these cases appear to me to deserve more atten- 
tion than they have yet received, I shall not build any argument on them. 
Recently, however, Prof. Wagner, of Kazan, has discovered that, among certain small 
gnats, the larvæ do not themselves directly produce the perfect insect, but give rise t0 
other larvæ, which undergo metamorphoses of 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 characterized 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 larvæ which do not now breed, in the course of ages may come to do so. 
If this idea is correct, it shows us how the remarkable phenomenon known as alterni 
tion 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 0? in 
other. In the wingless species of Orthoptera there is little difference, excepting in 8/5 
between the young larva and the perfect insect. The growth is as simple and gradual is 
in any other animal; and the creature goes through nothing which would, in ordinal) 
language, be called a metamorphosis. In the majority of Orthoptera the prese 
wings produces a marked difference between the larva and the imago. The habits, how: 
ever, are the same throughout life, and consequently the action of external circumstant? 
affects the larva in the same manner as the perfect insect. 
This is not the case with the Ephemeridæ. The larv do not live under the same gi 
ditions as the perfect insects; external forces accordingly affect them in a differ? 
manner ; and we have seen that they pass through some changes which bear no referen” 
to the form of the perfect insect: these changes, however, are for the most p 
gradual. The caterpillars of Lepidoptera have even more extensive changes t0 
rd the larva, for instance, is remarkably unlike that of the perfect! 
5° m this organ, however, could hardly take place while the insect was still 
nsect. 
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