NATURAL HISTORY OF CREATION. 165 



perhaps, it is only because there is no longer a possi- 

 bihty, in the higher ty2)es of being, of giving sufficiently 

 favourable conditions to carry on species to species, that 

 we see the operation of the law so far limited. 



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 

 farther 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 iish-form. 

 Such defects are the result of nothing more than a 

 failure of the power of development in the system of the 

 mother, occasioned by weak health or misery, and bearing 

 with force upon that sub-stage of the gestation at which 

 the perfecting of the heart to its i-iglit form ought 

 properly to have taken place. Here we have apparently 

 a realisation of the converse of those conditions which 

 carry on species to species, so far, at least, as one organ 

 is concerned. Seeing a complete specific i-etrogression in 

 this one point, how easy it is to suj^jDOse an access of 

 favourable conditions sufficient to reverse the phenome- 

 non, and make 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 in the measure 

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

 natural an occurrence as the other) would suffice in a 

 goose to give its progeny the body of a rat, and produce 

 the ornithorhynchus, or might give the progeny of an 

 ornithorhynchus the mouth and feet of a true rodent, 

 and thus complete at two stages the passage from the 

 aves to the mammalia. 



Perhaps even the transition from species to species 

 does still take place in some of the obscurer fields of 



