CROCODILES AND SNAKES 
267 
which shall apply to all alike. We have already seen what a wonder¬ 
ful difference there is between such mammals as the bat and the lion, 
the monkey and the whale, or the elephant and the mouse; but even 
between creatures so very unlike one another as these, there is still a 
great resemblance in many important ways. They all, for instance, 
BLACK CROCODILE 
possess four limbs, and their young, with one single exception, are all 
born alive, while they always breathe air itself, and never respire 
water by means of gills. 
But in the case of the Reptiles we find no such rules as these. 
Some have four legs, and others none. Some lay eggs, and some pro¬ 
duce living young. Some breathe air, and some breathe water, so that 
it is really almost impossible to draw up a description which shall be 
