26 THE BRITISH NATURALIST. [February 



though this effect may be far too sUght to be measured. Much of the 

 difficulty attending the study of variations amongst living organism.s 

 arises from the complexity of the definite, though minute, influences 

 of the environment, which produce definite, though minute, effects. 

 The position of an egg in a nest, for instance, will have some effect 

 upon a chick hatched from it. One egg may get a little more or a 

 little less warmth than another, and this must exert some influence on 

 the developement of the embryo. It is well to bear in mind these 

 difficulties attending the study of variation, even amongst animals 

 that we can closely observe, for we have reason to suppose that early 

 in the secondary period variations amongst reptiles would go on more 

 rapidly than to-day, because the only other terrestrial vertebrates were 

 a few small mammals of the marsupial group, whose descent can be 

 traced from Batrachian ancestors. 



That amongst these variations some were in an ornithic direction 

 there can be no doubt ; but before going further into this matter, I 

 would point ont the striking resemblances — both morphological and 

 physiological — which force all competent observers to recognize the 

 intimate relationship between reptiles and birds, and have induced 

 systematic naturalists to place them in one group, viz. : the Sauropsida. 



In the skeleton of all Sauropsidans the skull articulates wdth the 

 vertebral column by a single articulating surface or condyle. In this 

 respect they differ from other vertebrates, for both in the lower group, 

 Batrachians, and the higher group, the mammals, the skull articulates 

 by two condyles. 



A further striking feature of the Sauropsidan skeleton is the lower 

 jaw, each ramus, or half of which, is composed of several pieces, and 

 articulates with the skull, not directly, but by the intervention of a 

 peculiar bone called the quadrate. 



Amongst the physiological characters of Sauropsidans, perhaps the 

 most striking fact is that in all, the red-blood corpuscles are oval 

 nucleated discs. It is true that the colored blood corpuscles of the 

 Batrachians are also nucleated and oval ; but it must be remembered 

 that the largest colored corpuscle of the largest reptile is very much 

 smaller than the smallest colored corpuscle of the smallest Batrachian; 

 and though the temperature of birds is very much higher than that of 

 reptiles, higher even than that of mammals, yet the close similarity, 

 both in size and shape of the red corpuscles, indicates a close relation- 



