14 INTRODUCTION 



function when applied to animals before they reach the adult 

 condition is known as Embryology or Development. From very 

 early times it attracted a considerable amount of attention, but was 

 first placed on a really scientific footing by von Baer (1792-1876), 

 and as the result of the labours of many specialists, notably Francis 

 Maitland Balfour (1851-82), formerly Professor of Animal 

 Morphology in the University of Cambridge, and the Russian 

 zoologist Kowalewsky, is now an important, perhaps even an 

 overestimated, branch of the subject. As in so many other cases, 

 the interest of the facts has been enormously increased as a result 

 of the influence of the theory of evolution. Consisting as the 

 subject-matter does of the life-histories of animals, it has always 

 formed part of the work of the field naturalist, who has delighted 

 in the mysterious growth of frog from tadpole, or the passage of 

 caterpillar into chrysalis, and that again into the butterfly. Such 

 interest had quite a new turn imparted to it when the evolutionist 

 formulated the " Law of Recapitulation ", according to which, in 

 the words of Milnes Marshall, ''every animal in its own develop- 

 ment repeats its history, climbs up its own genealogical tree ". 

 Thus interpreted, the fish-like tadpole points to the descent of 

 frogs from ancestors which in many respects possessed those 

 characters which we now associate with fishes. There has been, 

 however, a tendency to exaggerate the importance of this law. 



6. The Stand-point of Distribution. Under this heading two 

 things call for consideration (a) Distribution in Space and (b] 

 Distribution in Time. 



(a) Distribution in Space. Everyone knows that different coun- 

 tries are inhabited by different sorts of animals, e.g. that kangaroos 

 are to be found in Australia, ostriches in Africa, and sloths in 

 South America. Until the rise of the evolution theory no ex- 

 planation was given of such facts, except that animals have been 

 created separately where we now find them. This was the 

 doctrine of "special creation", and if we adopt it, the study of 

 distribution can be little more than a recording of facts. Admitting 

 however, the truth of the evolution theory, it becomes possible to 

 give a reasonable explanation of many curious and at first sight 

 anomalous instances of the sort. We have, for example, the well- 

 known case of the tapir, an animal somewhat resembling the pig in 

 appearance but possessing a short proboscis. These creatures are 

 only found in South America and the Malay region. We know, 



