THE EVIDENCE FROM DEVELOPMENT. 171 



lopment," which may be briefly expressed in the phrase that 

 " development proceeds from the general to the special." 



To rightly understand the purport of this axiom, our preliminary 

 studies in the constitution of the animal kingdom must be borne in 

 mind. The animal world, as we have seen, is divided into a number of 

 great types, the most consistent and most firmly established of which 

 are the Vertebrates, including the " backboned " animals from fishes 

 to man ; Molluscs, including shell fish, such as oysters, cockles, snails, 

 and cuttle-fishes ; and Annulose animals, or Articulates, represented 

 by animals with jointed bodies, such as worms, insects, centipedes, 

 crustaceans, &c. Now, if the development of a number of animals 

 belonging to any one of these three divisions is observed, the egg of 

 each animal is seen to pursue the even tenor of its way, and to pass at 

 first through exactly the same stages of development. Up to a given 

 point, the stages in the development of all Vertebrates, for example, 

 are essentially similar. Sooner or later, however, development begins 

 to specialise its course, and hence arise the differences which mark 

 the adult forms. So also with Molluscs, which in their earlier stages 

 pass through essentially the same changes, but sooner or later diverge 

 in their course; each organism passing on its own way to assume 

 the special features which characterise the adult. Such was the im- 

 portant generalisation of Von Baer. Succeeding research has but 

 tended to establish Von Baer's doctrine, whilst it has also enlarged 

 the conception he was the first to promulgate. It is now known that 

 there are stages in early development which are common to the eggs 

 or germs of all animals alike ; and that, from the common track thus 

 pursued up to a given point by animal life at large, each group of 

 animals ultimately diverges on its own special way of life. Von 

 Baer's generalisation has thus come to include the whole animal 

 world, and has in recent times tended powerfully to support the' 

 doctrine which would explain the origin of living beings by presuming 

 their descent from pre-existing forms, and their relation with each 

 other as twigs, boughs, and branches of a great connected tree. 



The relation of development and its study to the hypothesis of 

 evolution is thus easy of determination. It is a perfectly reasonable 

 and most natural conception that in the development of a living being 

 we should obtain some clue to the history of its origin and to the 

 birth of its race. If its origin be a subject of research at all, any 

 information concerning the stages through which an animal or plant 

 becomes the adult organism, and by which it advances from literal 

 non-existence to the staid solidity of mature form and perfect life, 

 should, by analogy the most natural, be regarded as a veritable mine 

 of knowledge concerning its own beginning and, by further analogy, 

 regarding the beginnings of the world of life at large. Upon such a 

 thought is founded the dependence which modern biology is led to 



