6 ZOOLOGY 



process of which no explanation could or ought to be offered. 

 Nevertheless it was obvious that a Jackal was far more like a 

 Wolf than either of them was like a Tiger, and that in a natural 

 system of classification this fact should be expressed by placing the 

 Wolf and Jackal in one family, the Tiger in another. 



All through the animal kingdom the same thing occurs : no 

 matter what group we take, we find the species composing it 

 resemble one another in varying degrees, or, as it is sometimes ex- 

 pressed, have varying degrees of relationship to one another. On 

 the view that each species was separately created the word relation- 

 ship was used in a purely metaphorical sense, as there could of 

 course be no real relationship between two groups of animals 

 having a totally independent origin. But it was assumed that 

 creation had taken place according to a certain scheme in the 

 Divine Mind, and that the various species had their places in this 

 scheme like the bits of glass in a mosaic. The problem of classifica- 

 tion was thus to discover the place of each species in the pattern of 

 the unknown design. 



The point of view underwent a complete change when, after the 

 publication of Darwin's Origin of Species in 1859, the Doctrine 

 of Descent or of Organic Evolution came to be generally 

 accepted by biologists. A species is now looked upon, not as an 

 independent creation, but as having been derived by a natural 

 process of descent from some pre-existing species, just as the 

 various breeds of Domestic Fowl are descended from the little 

 Jungle-fowl of India. On this view the resemblances between 

 species referred to above are actually matters of relationship, and 

 species are truly allied to one another in varying degrees since 

 they are descended from a common ancestor. Thus a natural 

 classification becomes a genealogical tree, and the problem of 

 classification is the tracing of its branches. 



This, however, is a matter of extreme difficulty. Kepresenting 

 by a tree the whole of the animals which have ever lived on the 

 earth, those existing at the present day would be figured by the 

 topmost twigs, the trunk and main branches representing extinct 

 forms. Thus the task of arranging animals according to their 

 relationships would be an almost hopeless one but for two 

 circumstances : one, that remains of many extinct forms have been 

 preserved ; the other, that the series of changes undergone by an 

 animal in its development from the egg sometimes appears to afford 

 an indication of the changes by which, in the course of ages, it has 

 been evolved from an ancestral type. Evidence furnished by the 

 last-named circumstance is, of course, furnished by embryology : the 

 study of extinct animals constitutes a special branch of morphology 

 to which the name Palaeontology is applied. 



The solid crust of the earth is composed of various kinds of 

 rocks divisible into two groups : (1) Igneous rocks, such as granite 



