4 ORTHOGENETIC EVOLUTION IN PIGEONS. 



realm. How could teleological results flow from non-teleological causes? That was 

 the staggering problem that confronted Darwin and kept him from feeling entirely 

 convinced "two or three years" longer of the mutability of species. Already he 

 had "perceived that selection was the keystone of man's success in making useful 

 races of animals and plants; but how selection could be applied to organisms living 

 in a state of nature remained for some time a mystery."* For clearing up this 

 mystery the credit still belongs to Darwin, for he alone knew there was such a 

 mystery and made the application that dissolved it. As Huxley remarks: 4 



Variation occurs under natural, no less than under artificial, conditions. Unrestricted 

 multiplication implies the competition of varieties and the selection of those which are 

 relatively best adapted to these conditions. 



This is a concise statement of the theory of natural selection, so simple and 

 self-evident that it at once carries conviction. The theory tells us that nature 

 is self-regulating in the living as in the non-living world. It tells us that the more 

 useful variations have the best chance of survival in the struggle for existence, 

 and thus shows that, variation being given, progressive evolution is inevitable. 



The theory does not undertake to explain the first appearance of life or to state 

 the causes of variation, but it does show how nature sifts out the fit from the unfit 

 and builds upon such variations as have a winning value in the battle of life. 



For this all-embracing viewpoint, which completes and crowns the advances 

 by Newton and Lyell, the world is indebted to Charles Darwin. 



The theory of de Vries, the distinguished author of the mutation theory, 

 coincides in the main with that of Darwin; but he introduces a distinction in regard 

 to variations which he regards as fundamental, dividing them into two classes: 

 (1) ordinary or fluctuating variations, and (2) mutations. All species exhibit "fluc- 

 tuations," but these obey Que"telet's law of probability and never transgress the 

 limits of the type. From them improved races may result, but never new species. 



Mutations, on the other hand, are those variations which remain constant, 

 and these are the sole source of new species and new varieties. Fluctuations are 

 regarded as merely quantitative, while mutations stand for qualitative changes. 

 However small or large, mutations are always sudden in appearance; that is, they 

 do not arise by slow and transitional degrees, but come ready-made ; and although 

 they are subject to fluctuations, they are essentially immutable units the so-called 

 "unit-characters." A species thus consists in some definite number of these fixed 

 units, from the germ onward to the full adult stage, and this number remains con- 

 stant. The loss or addition of a single unit would make at once a new species. 



The fundamentals in the mutation theory are: 



1. Every species consists of a fixed number of unit-characters. 



2. The species and the component units are alike sudden in origin and unchanging 



in type. 



3. Old characters may be suddenly transmuted into new ones, but between the two 



there is always a gulf of absolute discontinuity, with no possible bridge of 

 modification. 



4. Continuous intergradations may connect a species with an improved race, but 



never one species with another species. 



1 Life and Letters, i, p. 68. 4 Darwiniana Essays, p. 279. 



