130 HYPOTHESIS OF THE DEVELOPMENT OF 



the result of the operation of the law of development in the 

 generative system. 



Let us trace this law also in the production of certain 

 classes of monstrosities. A human foetus is often left with 

 one of the most important parts of its frame imperfectly 

 developed ; the heart, for instance, goes no further than the 

 three- chambered 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 realiza- 

 tion 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 pro- 

 cess, reversing the phenomenon, and making a fish mother 

 develop a reptile heart, or a reptile mother develop a mammal 

 one. It is no great boldness 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 orni- 

 thorhynchus, 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 par- 

 ticulars 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 count- 

 less instances every spring is, in part at least, as thoroughly 

 a transmutation of the fish organization into that of the reptile, 

 as the supposable change of sauroid fishes into saurian reptiles 

 could ever be. It is different, as being only a process in ordi- 

 nary 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 : 



