cH. XVIII The Evolution of Evolution Theories 299 



has championed the theory of natural selection with more 

 confident consistency or with more skilfully handled 

 weapons. 



8. The Present State of Opinion. — As Wallace says in 

 the preface to his work on Darwinism, " Descent with 

 modification is now universally accepted as the order of 

 nature in the organic world." But, while this is true, there 

 remains much uncertainty in regard to the way in which the 

 progressive ascent of life has come about, as to the mechan- 

 ism of the great nature-loom. The relative importance of 

 the various factors in evolution is very uncertain. ^ 



The condition of evolution is variability, or the tendency 

 which animals have to change. The primary factors of 

 evolution are those which produce variations, which cause 

 organic inequilibrium. Darwin spoke of variations as 

 " fortuitous," " indefinite," " spontaneous," etc., and frankly 

 confessed that he could not explain how most of them arose. 



Ultimately all variations in organisms must be due to 

 variations in their environment, that is to say, to changes in 

 the system of which organisms form a part. But this is 

 only a general truism. 



' All naturalists, however uncertain in regard to tlie factors in 

 evolution, accept the doctrine of descent — the general conception of 

 evolution — as a theory which has justified itself. It is not indeed so 

 demonstrable as is the doctrine of the conservation of energy, but it is 

 almost as confidently accepted. Few naturalists, however, have 

 attempted any philosophical justification of their belief. This is strange, 

 since it should surely give pause to the dogmatic evolutionist to reflect 

 that his own theory has been evolved like other beliefs, that his 

 scientific demonstration of it rests upon assumptions which have also 

 been evolved, that the entire system of evolutionary thought must be a 

 phase in the development of opinion, that, in short, he cannot be 

 dogmatic without being self-contradictory. See A. J. Balfour's Defence 

 of Philosophic Dovtt, pp. 260-274 (London, 1879). In regard to the 

 philosophical aspects of the doctrine of evolution see Prof. Knight's 

 essay on " Ethical Philosophy and Evolution " in his Studies in Philo- 

 sophy and Literature i^Lxm6., 1879), and, with additions, in Essays in 

 Philosophy {Boston and New York, 1890) ; Prof. St. George Mivart's 

 Contemporary Evolution (Lond. 1876) ; E. von Hartmann's Wahrheit 

 und Irrthum im Darwinismus ; an article by Prof. Tyndall on ' ' Vir- 

 chow and Evolution" in Nineteenth Century, Nov. 1878 ; and articles 

 on " Evolution" by Huxley and Sully in Encyclopedia Britannicq. 



