Among the Ophiurans also we find two well-marked types of development. Some passing through metamorphoses, while others, as for instance Ophiopholis bellis, “is developed very much after the method of Asteracanthion Mülleri, without passing through the Plutean stage.”[36]
Even in the same species of Echinoderm the degree of development attained by the larva differs to a certain extent according to the temperature, the supply of food, &c. Thus in Comatula, specimens which are liberally supplied with sea-water, and kept warm, hurry as it were through their early stages, and the free larva becomes distorted by the growing Pentacrinus (see Fig. [43]), almost before it has attained its perfect form. On the other hand, under less favourable conditions, if the temperature is low and food less abundant, the early stages are prolonged, the larva is longer lived, and reaches a much higher degree of independent development. Similar differences occur in the development of other animals, as for instance, in the Hydroids,[37] and among the insects themselves, in Flies;[38] and it is obvious that these facts throw much light on the nature and origin of the metamorphoses of insects, which subject we shall now proceed to consider.
CHAPTER IV.
ON THE ORIGIN OF METAMORPHOSES.
The question still remains, Why do insects pass through metamorphoses? Messrs. Kirby and Spence tell us they “can only answer that such is the will of the Creator;”[39] this, however, is a general confession of faith, not an explanation of metamorphoses. So indeed they themselves appear to have felt; for they immediately proceed to make a suggestion. “Yet one reason,” they say, “for this conformation may be hazarded. A very important part assigned to insects in the economy of nature, as we shall hereafter show, is that of speedily removing superabundant and decaying animal and vegetable matter. For such agents an insatiable voracity is an indispensable qualification, and not less so unusual powers of multiplication. But these faculties are in a great degree incompatible; an insect occupied in the work of reproduction could not continue its voracious feeding. Its life, therefore, after leaving the egg, is divided into three stages.”
But there are some insects—as, for instance, the Aphides—which certainly are not among the least voracious, and which grow and breed at the same time. There are also many scavengers among other groups of animals—such, for instance, as the dog, the pig, and the vulture—which undergo no metamorphosis.
It is certainly true that, as a general rule, growth and reproduction do not occur together; and it follows, almost as a necessary consequence, that in such cases the first must precede the second. But this has no immediate connection with the occurrence of metamorphoses. The question is not, why an insect does not generally begin to breed until it has ceased to grow, but why, in attaining to its perfect form, it passes through such remarkable changes; why these changes are so sudden and apparently violent; and why they are so often closed by a state of immobility—that of the chrysalis or pupa; for undoubtedly the quiescent and death-like condition of the pupa is one of the most remarkable phenomena of insect-metamorphoses.
In the first place, it must be observed that many animals which differ considerably in their mature state, resemble one another more nearly when young. Thus birds of the same genus, or of closely allied genera, which, when mature, differ much in colour, are often very similarly coloured when young. The young of the lion and the puma are often striped, and the fœtal Black whale has teeth, like its ally the Sperm whale.