EVOLUTION AND ITS CONSEQUENCES. X7 



which distinguish all of the higher groups of animals and of plants 

 among themselves. It is easier to ask than to answer what advan- 

 tages the mammalian skeleton possesses over the reptilian that it 

 should have superseded it. What end was served by aborting 

 the coracoid bone, which in reptiles supports the shoulder-joint 

 from behind, answering to the ischium of the pelvis ? I do not 

 know how to answer this question on a teleological basis, although 

 it involves one of the principal characters of the class of reptiles. 

 What mechanical end was gained by withdrawing the rib-bones 

 of two cranial segments into the cavity of the ear, to become the 

 hammer, anvil, and stirrup of the organ of hearing ? * Was it to 

 perfect the auditory faculty ? Scarcely ; for birds possess as re- 

 fined and as musical an ear as any mammals, and appear to be 

 superior to them in discriminating power, yet in them the ham- 

 mer is the basal element of the lower jaw, and the anvil supports 

 it, being entirely outside the cranial walls. See again one ground 

 of distinction between reptiles and batrachians. The base of the 

 brain-case in the former consists of an axis of bony segments de- 

 veloped in the primordial cartilage, while in the batrachian it is a 

 single bony plate, formed by deposit in the membrane which origi- 

 nally bounded this cartilage. Who can assign any advantage of the 

 one type above the other which can be looked upon as m any way 

 related to the external needs of the animals of those classes ? 



Another example may be found in the ankle-joints of reptiles, 

 birds, and mammals. In the first two the hinge is between the 

 first and second series of tarsal bones ; in the mammals, between 

 the bones of the lower leg and the first row of tarsal bones. Some- 

 thing besides the superior mechanical advantages of the latter has 

 given it predominance over the former. 



To turn to the nautilus and ammonite types of Mollusca, we 

 observe beautiful illustrations of all the laws already stated. As 

 is well known, these shells have their tubular cavity divided by 

 transverse partitions. In the nautilus these unite with the outer 

 wall by a plain angle, but in various genera which lead toward the 

 ammonites this margin becomes complicated. This results from 

 an excessive growth of the peripheral part- of the partition or sep- 

 tum, so that in order to confine it to the same space of contact it 

 must be folded. This plication takes place in a symmetrical man- 

 ner. The folded edge in Aturia forms a tongue-like loop on each 



* The homoloj^ics here expressed have been rendered improbable by late investi- 

 gation. The argument is, however, not affected. 



