n6 THE BOOK OF NATURE STUDY 



speak, more observable habits than most fishes have ; the skull 

 of a cat or dog is a good subject for a school lesson, but that of a 

 cod or flounder is not, and so on. The question is, what can be 

 done most effectively with different kinds of material. 



In regard to reptiles, we would suggest a study of a living 

 grass snake, especially of one that has grown accustomed to 

 being a pet. The study should have this particular object in 

 view, to illustrate adaptation, i.e. how peculiarities of structure are 

 fitted for peculiar conditions of life. 



1. One may start from the fact that the body is greatly elon- 

 gated. The trunk portion is relatively long, while the tail 

 portion (without ribs on the vertebrae) is relatively short. Limbs 

 are inconsistent with such a long body, and with a habit of creep- 

 ing through brushwood and crevices of all sorts. Limbs and 

 limb girdles have disappeared, save small vestiges of the hip girdle 

 and even of hind-legs in a few cases, e.g. in boas and pythons. 

 Technically expressed, the absence of limbs is correlated with 

 the great elongation of the body. But it must be made clear 

 that the elongation is in the trunk region of the body ; it is not a 

 matter of having a very long tail, as many lizards have. 



2. The absence of limbs is compensated for by the presence of 

 long ribs on all the precaudal vertebrae (sometimes nearly three 

 hundred) except the first. The vertebrae of the tail have long 

 transverse processes. The ribs are very movable forwards and 

 backwards, and their lower ends are fastened to the large ventral 

 scales which grip the roughnesses on the ground. 



The snake, Ruskin says, " literally rows on the earth, with 

 every scale for an oar ; it bites the dust with the ridges of its 

 body." The comparison with rowing is luminous, but perhaps 

 it would be truer to call the ribs the oars and the ventral scales 

 their blades. On a perfectly smooth surface it can make no 

 headway, but in normal conditions the edges of a certain number 

 of scales are fixed against roughnesses of the ground, the ribs 

 are drawn together first on one side, then on another, the body 

 is thus wriggled forward to the place of attachment, the front 

 part shoots out as the hind part fixes itself, an anterior attach- 

 ment is again effected, and thus the snake flows on. And what 

 a wonderful locomotion it is ! For the limbless serpent, as 



