THE LIVING WORLD 241 



to a greater or less extent in the development of almost 

 all animals. 



The embryos of higher animals for the most part 

 transitorily resemble, in their general features, the 

 structure of other animals lower in the scale. The series 

 of forms, also, through which the embryo of a higher 

 animal passes in its development (or ontogeny), succes- 

 sively resembles, in a general way, a series of adult forms 

 of animals lower in the scale of life. 



Thus the heart of a cat is at first but a single tube, as 

 it permanently remains in the ascidians or sea squirts. 

 The cat's brain consists in its earliest stages of a series of 

 simple vesicles, roughly like the brain of that lowly fish, 

 the lamprey. In a more advanced stage, the embryo of 

 the cat is plainly the embryo of a beast not of a fish 

 and later on it is plainly that of some beast of prey. 



Most remarkably obvious are those changes which take 

 place when the embryo is a free active creature during 

 its development. 



Thus the young of the frog is, as before said, a 

 tadpole, and so the frog in its development is said to 

 undergo a metamorphosis. No less marked is that change 

 which most insects undergo, and which is so well seen in 

 butterflies and moths, which are first actively feeding 

 grubs, then quiescent crysalides, and finally reproductive, 

 usually winged, adults. These stages are known in 

 zoology as (i) the larva ; (2) the pupa ; and (3) the imago 

 or the perfect and mature insect. 



Thus the whole of the living organic world begins, as 

 it were, from a common unicellular starting-point, whence 

 the two kingdoms of organic life may be said to diverge. 

 The animal kingdom advances in complexity from a 

 structure resembling a double-walled sack, with a per- 

 manent digestive cavity, and possessing nervous and 



