l] ORIGIN OF SPECIES 11 



The distinctions between species are of very different degrees, 

 so that for convenience species closely resembling 



Classification. 



each other are collected into genera genera into 

 families families into orders orders into classes and classes 

 into phyla. These are the names in commonest use, but often the 

 nature of the subject requires the introduction of further grades of 

 difference, and the number of grades actually employed depends to 

 a large extent on the point to which the analysis is pushed. 



The only theory of the origin of species which has so far 



commanded any considerable agreement amongst 



naturalists is the famous theory of Charles Darwin. 



According to this theory, the resemblances between 

 a number of living species are due to the fact that these species 

 are descended from a common ancestral species which possessed the 

 common features as characters of its own. Therefore, the degree of 

 likeness between species is the expression of a nearer or remoter 

 blood relationship, and it logically follows that, since no part of 

 the animal kingdom is without resemblances to the rest, if we 

 recede far enough in time we reach a period when all the animals 

 in the world constituted one species. 



To a certain extent Darwin's theory was only the expression of 

 ideas that had first occurred to Greek philosophers, and had in one 

 form or other been put forward by many naturalists before him. 

 His special merit lies in that he pointed out various processes 

 at present going on in nature which must lead to the modification 

 of species. He recalled attention to the well-known fact which we 

 have just discussed, that although the offspring in general resemble 

 the parents, yet this resemblance is never exact, and further that 

 the young of one brood often differ quite perceptibly from one 

 another, and that these differences are often inherited by the off- 

 spring of the individuals showing them. 



Again, another fact well-known but usually ignored, was em- 

 phasised by Darwin : viz., that if the state of the animal population 

 of the globe remains fairly constant, out of all the young produced 

 by a pair of parents during their lifetime on an average only two 

 will survive, since if more were to live the species would inevitably 

 increase in numbers. Hence since each animal tends to multiply 

 at a rate at which if unchecked it would soon overrun the globe, a 

 competition must result between the members of each species both 

 for food and in the escape from enemies, as a result of which the 

 " fittest " will survive. So long as the surroundings of the species 



