152 HYPOTHESIS OF THE DEVELOPMENT OF 



in so many instances ? Admit their explanation, that a mere 

 mistake has been made in calling that species which was only 

 variety, what guarantee can we have for the fixity of any so- 

 called species, when it has given way in such instances 1 What 

 is species, if it cannot be fixed upon such a vast assemblage as 

 the Thallogens, or even the progeny of the Thelephora sul- 

 phurea ? Of the believers in species as a fixed thing, can we 

 now believe that they know anything about it ? Apart from 

 all theorizing about the absolute character of the thing called 

 species, do not these facts show a transibility and intercom- 

 munion of forms totally at variance with those general opinions 

 as to fixity which now reign in the scientific world ? 



In the animal kingdom, we have fewer illustrations of modi- 

 fiability of transition ; but they tend to exactly the same effect. 

 I shall here pass over the succession of forms which appears in 

 common infusions. Neither shall I enter into the particulars 

 of a late curious investigation by a Danish naturalist, which 

 results in showing alternative forms in the succession of certain 

 animals low in the scale, including the medusa ; that is, as it 

 were, A giving birth to B, B to C, and C to A again. 1 Such 

 matters are as yet obscure, however highly they may promise 

 in time to illustrate this question. Let us rather look to de- 

 partments of this kingdom which come broadly under the ob- 

 servation of naturalists. In the mollusca there occurs a modi- 

 fiability of a most remarkable nature. Fresh- water species of 

 these, exposed to brackish water, assume, where able to survive 

 the change, characters in the exterior form of the shell proper 

 to their marine congeners, and involving differences from the 

 original animal muck greater than is usually sufficient with 

 naturalists to constitute a distinction of species, if not of tribe or 

 family. Many years ago, Pennant remarked the singular 

 modification of stomach which the common trout appears to 

 have undergone in the lakes of the county of Galway, in con- 

 sequence of feeding on shell-fish. The integument has become 

 as thick as the gizzard of a bird, manifestly in consequence of 

 an effort of nature to accommodate herself to the peculiar food 

 of the animal. So also, when a common gull was fed upon 

 corn, the parietes of the stomach were found, on examination 

 after death, to be thickened.' 2 The peculiar forms of the man- 

 dibles of birds are grounds of specific distinction j yet it is 



1 Steinstrup on Alternate Generation, published by the Hay Society. 



2 Yarrell's Birds, iii. 571. 



