ANIMAL METAMORPHOSIS. 183 



the study of them. In a general way, it may be stated that meta- 

 morphosis gives its possessor an advantage in " the struggle for 

 existence," or assists it in carrying out the fiat of the Creator, "to 

 be fruitful and multiply." In some cases it seems to be a provi- 

 sion for the distribution of species. Thus, many of the lawer 

 animals which in adult life are rooted to one spot — as the Hydra, 

 Sponges, and Corals — or whose powers of movement are very 

 limited — as the Echinodermata, MoUusca, etc. — by means of 

 their free-swimming embryos, are enabled, either by their own 

 locomotive powers or by the action of waves and currents, to be 

 carried to a distance from their birthplace, and so to be dissemi- 

 nated over the surface of the globe, and to prevent the spots 

 inhabited by their respective species from being overcrowded by 

 the accumulation of their progeny. In others, as the Anoda or 

 Fresh-water Mussels, the same object is attained by the larval 

 forms being provided with prehensile organs, by which they can 

 attach themselves, as parasites, to the gills and fins of fishes, and 

 in like manner be carried away from the abode of their parents ; 

 these organs being lost on their attaining adult life. 



Protection from various dangers to which the young are 

 exposed is another reason for metamorphosis, the form, shape, 

 and colour of the defenceless larvae being often adapted to the 

 food on which they subsist, so as to be better able to escape the 

 observation of their various enemies ; while the pupal stage 

 enables those which are unable to stand sudden changes of 

 weather and temperature to pass some part of their existence in a 

 condition of almost suppressed vitality, in which they are capable 

 of resisting a degree of cold or wet which would be fatal to the 

 adult form, and would speedily prevent the continuance of their 

 life upon the earth ; the hard and horny case of others prevents 

 their being a tempting morsel to the birds, which would otherwise 

 prey upon them. How small would be the chance of the survival 

 of butterflies, moths, frogs, etc., if they emerged from their minute 

 eggs simply as tiny representatives of their parents ! How little 

 would they be able to resist a sudden storm of biting east wind, 

 or even the downfall of a passing thunder-shower ! 



Coupled with the study of embryology, the facts of metamor- 

 phosis have thrown great light on the theory of evolution ; helping 

 VOL, jv, o 



