XIX.] MORPHOLOGY. 237 



large feet, and the reverse ; and they frequently present 



corresponding monstrosities. It seems impossible that this 



tendency of homologous parts to vary together can be in 



any way referable to the law of adaptation : I believe it 



has much more resemblance to the law, that the similar 



edges and angles of a crystal are similarly modified by the like simi- 



formation of secondary planes on them. In this case, as of'^crystals. 



in that of the nipples, I do not say, nor do I believe, that 



there is anything inconsistent with the law of adaptation 



to purpose. I only say that there are relations which are 



not cases of that law, and are not to be explained by it. 



But still more remarkable are the parallelisms between Only un- 

 parts which are not homologous. Thus, none but ungulate fj^^als 

 or hoofed animals have horns, though horns are not found iavehoms. 

 on aU the Ungulata. It cannot be that no other animals 

 have any need for them. Horns are weapons, to be used 

 in fighting, and would have been in the highest degree 

 useful to carnivorous animals, had they been endowed 

 with them. The only reason, I think, that can be sug- Eeason 

 gested why ungulates alone should have horns, is a reason suggested, 

 that does not in any way come under the law of adapta- 

 tion ; it is, that they have a tendency to the production of 

 horny matter, which appears m both the horns and the 

 hoofs. I do not say that this conjecture is proved, or 

 capable of proof; but it is supported by the fact that, 

 when a tissue normally exists in one part of an organism, 

 it is sometimes abnormally produced in another part, as 

 for instance when the muscular walls of the heart are 

 converted into a substance resembling bone. This, of 

 course, occurs only in disease ; but the tendency of the 

 Ungulata to produce horny matter in their hoofs may in 

 a somewhat similar manner determine its j)roduction in 

 their horns. But, though the law of adaptation will not, I 

 believe, account for the existence of horns on Ungulates 

 only, it does account for their growing on the head, and 



I not on any other part ; for it is only there they could be 

 useful. 

 But there is, again, a remarkable fact concerning the Position 

 position of horns on the head, which cannot, I think, ][,e°^'^°™^' 



