146 HYPOTHESIS OF THE DEVELOPMENT OF 



form, so that it is the heart of a reptile. There are even 

 instances of this organ being left in the two-chambered or fish- 

 form. Here we have apparently a realization of the converse 

 of advance of grade, so far, at least, as one organ is concerned. 

 Seeing a complete specific retrogression in one point, how easy 

 it is to suppose a simply natural process, reversing the pheno- 

 menon, and making a fish mother develop a reptile heart, or a, 

 reptile mother develop a mammal one. It is no great bold- 

 ness to surmise that a super-adequacy of force in the measure 

 of this under-adequacy (and the one thing seems as natural an 

 occurrence as the other) would suffice in a natatorial bird to 

 give it as a progeny the ornithorhynchus, or might give the 

 progeny of an ornithorhynchus the mouth and feet of a true 

 mammalian, and thus complete at two stages a passage from 

 one class to another. 



Perhaps with the bulk of men, even those devoted to science, 

 the great difficulty is, after all, in conceiving the particulars of 

 such a process as would be required to advance a fish into a 

 reptile. And yet no difficulty could well be less substantial, 

 seeing that the metamorphosis of the tadpole into the frog a 

 phenomenon presented to our observation in countless in- 

 stances every spring is, in part at least, as thoroughly a 

 transmutation of the fish organization into the reptile, as the 

 supposable change of sauroid fishes into saurian reptiles could 

 ever be. It is different, as being only a process in ordinary 

 generation ; but it realizes, as far as the necessary organic 

 changes are concerned, the hypothetic view of an advance of 

 one grade of animal forms into another. There is another fact 

 connected with the reproduction of the batrachian order of 

 reptiles, that, when the young are enclosed in a dark box sunk 

 in a river, with holes through which the water may flow, the 

 animals grow, but never undergo their destined change ; they 

 become gigantic tadpoles, and the reptile characters are not 

 developed. Here the progeny of a reptile may be said to be- 

 come a fish, though not a perfect one, and transition of 

 species is in a manner realized, although in retrogression. And 

 this is an instance in which the whole animal is concerned. 

 Now surely no one will deny that that which we see nature 

 undo she is able to do, and might be seen doing, if the pro- 

 per occasion were to occur, or were the requisite attendant 

 conditions realized. 



So much with regard to grade. Let us now consider the 

 principle of modifiability that part of the hypothesis to which. 



