108 
NATURE 
‘| Fune 5, 1873 ; 
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larva of a star-fish, and in 1847 MM. Koren and Daniel- 
son satisfied themselves that this was the case. 
Figs. 45 and 46 represent the front and side view of a 
Bipinnaria found by Muller* near Marseilles. a is the 
mouth, é the cesophagus, ¢ the stomach, <’ the intestine. 
Fig. 47 represents a somewhat older specimen in which 
the Starfish (4) is already beginning to make its appear- 
ance, , 
But while certain Starfishes thus go through metamor- 
phoses, similar in character to, and not less remarkable 
than, those of sea-eggs ; there are others, as, for instance, 
the genus Asteracanthium, in which the organs and 
appendages special to the Pseudembryo, are in abeyance, 
while in Pteraster “the zooid is reduced to an investing 
sheet of sarcode.” + 
Even in the same species the degree of development 
attained by the Jarva differs to a certain extent according 
to the state of the temperature, the supply of food, &c. 
Thus in Comatula, specimens which are liberally supplied 
with sea-water, and kept in a warm temperature, hurry as 
it were through their early stages, and the free larva 
becomes distorted by the growing Pentacrinus, 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 pro- 
longed, the larva is longer lived, and reaches a much 
higher degree of independent development. Weissmann 
has observed similar differences in the larve of Flies,t 
and it is obvious that these facts throw much light on the 
nature and origin of metamorphoses as we see them 
among insects, but the latter question we shall now pro- 
ceed to consider. 
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;”§ which, however, is rather a general 
confession of faith than an explanation of metamorphoses. 
And this they appear to have felt themselves ; for they 
immediately proceed to make a further 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 I 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 divi- 
ded into three stages.” 
But there are some insects, as, for instance, the Aphides, 
which certainly are notamong theleast 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 
1eproduction 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. And in addition to this, we must consider, 
first, the sudden and apparently violent nature of these 
transitions, and, secondly, the immobility of the animal 
in its pupa state; for undoubtedly the quiescent and 
* 1c Zweit. Abb. Pl, 1, Figs 8 and 9. : 
+ Thomson, on the Embryology of the Echinodermata, Natural History 
Review, 1863, p 4r5. 
t Zeits. fur Wiss. Zool, 1864, p. 228. 
§ Introduction to Etymology, 6th Ed. i, p. 6, 
deathlike condition of the pupa is one of the most re- 
markable characteristics of insect-metamorphosis, 
In the first place, it must be observed that many species 
which differ considerably in their mature state, agree 
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 similar when young. The 
young of the lion and the puma are often striped, and 
foetal whales have teeth. Leidy has shown that the milk 
teeth of the genus Aguus resemble the permanent teeth 
of the ancient Azchitherium, while the milk-teeth of 
Anchitherium again approximate to the dental system of 
the still earlier s/erychifpus. Rutimeyer, while calling 
attention to this interesting observation, adds that the 
milk-teeth of Eguus caballus in the same way, and still 
more those of 4. /ossz/is, resemble the permanent teeth 
of Hipparion. E 
In fact, the great majority of animals do go through 
well-marked metamorphoses, though in many cases they 
are passed through within the egg, and thus do 
not come within the popular ken. “La larve,” says 
Quatrefages, “ n’est qu’un embryon A vie independante.”* 
Those naturalists who accept in any form the theory of 
evolution, consider that “the embryonal state of each © 
species reproduces more or less completely the form and — 
structure of their Jess modified progenitors.”+ ‘Each 
organism,” says Herbert Spencer,f “exhibits within a 
short space of time a series of changes which, when <up- 
posed to occupy a period indefinitely great, and to go on 
in vaiious ways instead of one way, give us a tolerably” 
clear conception of organic evolution in general. =~ 
The naturalists of the older school do not, as Darwin 
and Fritz Muller have already pointed out, deny the facts, 
though they explain them in a different manner—generally 
by the existence of a supposed tendency to diverge from an 
original type. Thus Johannes Muller says “ the idea of de- 
velopment is not that of mere increase of size, but that of 
progress from what is not yet distinguished, but which 
potentially contains the distinction in itself, to the actually 
distinct,—it is clear that the less an organ is developed, 
so much the more does it approach the ty pe, and that; 
during its development, it more and more acquires pe- 
culiarities. The types discovered by comparative anatomy ~ 
and developmental history must therefore agree.” . ; 
And again, “ What is true in this idea is, that every 
embryo at first bears only the type of its section, from 
which the type of the class, order, &c., is only afterwards 
developed.” 
Agassiz also observes that “the embryos of different 
animals resemble each other the more the younger they 
are.” There are, no doubt, cases in which the earlier 
states are rapidly passed through, or but obscurely indi- 
cated ; yet we may almost state it as a general propo- 
sition, that, whether before or after birth, animals 
undergo metamorphoses. The maturity of the young 
animal at birth varies immensely. The kangaroo (Zacro- 
pus major), which attains a height of seven feet, ten 
inches, does not when born exceed one inch and two lines 
in length; the chick leaves the egg in a much more ad- 
vanced condition than the thrush ; and so among insects 
the young cricket is much more advanced, when it leaves 
the egg, than the fly or the bee ; and it is a familiar fact, 
that in this respect, though not of course to anything like 
the same extent, differences occur even within the limit of 
one species. 
In oviparous animals the condition of the young at 
birth depends much on the size of the egg; where the 
egg is large, the abundant supply of nourishment enables 
the embryo to attain a higher stage of development ; 
where the egg is small, and the yolk consequently scanty, 
itis soon exhausted, and the embryo requires an addi- 
* Metamorphoses de l’Homme et des Animaux, p. 133+ 
+ Darwin, Origin of Species, 4th Ed, p. 532. 5 
} Principles of Biology, vi. p. 349. 
