OKIGIN OF THE SPECIFIC TYPE 301 



must be different in a high tower-like structure and a low flat spiral ; 

 possibly, too, of the varied obstacles and resistance the snail has to 

 encounter in creeping into clefts and holes, or among a tangle of 

 plants, according to the form of its shell ; but is it not also conceivable 

 that the form of the shell has been determined by its contents 1 As 

 Rudolph Leuckart taught, the snail may be regarded as composed 

 of two parts, one of which is formed by the head and foot, the other 

 by the so-called ' visceral sac ' : the former may be called the animal 

 half, because it chiefly contains the dominant organs — the nerve- 

 centres, almost the whole mass of muscle, and the sense-organs ; 

 the latter the vegetative half, since it contains the main mass of the 

 nutritive and reproductive systems — the stomach and intestine, the 

 large liver, the heart, the kidneys, the reproductive organs, and so on. 

 The vegetative half of the animal is always concealed within the 

 shell ; would not therefore any great variation in the size of liver, 

 stomach, intestine, and so on, bring with it a variation in the size 

 and form of the shell, as well as in the expansion or contraction 

 of its coils? And might not such variations become necessary 

 because of some change in the food-supply ? It is only a supposition, 

 but it seems to me very probable that becoming accustomed to a new 

 diet, less easily broken up and dissolved and of diminished nutritive 

 value, would cause modification not only of the radula and jaw-plate, 

 but also of the stomach and the liver, the intestine and the kidneys, 

 whose activity is closely associated. The stomach must become more 

 voluminous, the liver which yields the digestive fluid must become 

 more massive, and so forth. I will not follow this hypothethical 

 example further, for I merely wished to recall the fact that the snail 

 shell, to the form of which no biological significance can be commonly 

 attributed, is actually a sort of external cast of the visceral sac, and 

 consequently dependent on the variations to which that is liable in 

 accordance with the conditions of its life. To give precise proofs 

 for such processes is certainly not yet possiljle, for we do not even 

 know with certainty what the diet of the various species of snail 

 is, much less the difference between the modes of nutrition in two 

 varieties, or the nutritive value of the materials used, or the 

 changes in secretion, absorption, assimilation, and excretion which 

 must be brought about by these differences. But we can at least 

 see that variations in nvitrition must be enouo-h in themselves to 

 give rise to new adaptations in the size, constitution, and mutual 

 adaptation of the internal vegetative organs, and we cannot overlook 

 the possibility that the form and size of the vegetative half, and 

 therefore the form and size of its secretion, the shell, may also be 



