Seite - 257 - in The Origin of Species

THEORY OF NATURAL SELECTION 257 species, to which it would apparently have been advantage- ous? But it is unreasonable to expect a precise answer to such questions, considering our ignorance of the past history of each species, and of the conditions which at the present day determine its numbers and range. In most cases only general reasons, but in some few cases special reasons, can be assigned. Thus to adapt a species to new habits of life, many co-ordinated modifications are almost indispensable, and it may often have happened that the requisite parts did not vary in the right manner or to the right degree. Many species must have been prevented from increasing in numbers through destructive agencies, which stood in no relation to certain structures, which we imagine would have been gained through natural selection from appearing to us advantageous to the species. In this case, as the struggle for life did not depend on such structures, they could not have been acquired through natural selection. In many cases complex and long- enduring conditions, often of a peculiar nature, are necessary for the development of a structure ; and the requisite con- ditions may seldom have concurred. The belief that any given structure, which we think, often erroneously, would have been beneficial to a species, would have been gained under all circumstances through natural selection, is opposed to what we can understand of its manner of action. Mr. Mivart does not deny that natural selection has effected something; but he considers it as "demonstrably insufficient" to account for the phenomena which I explain by its agency. His chief arguments have now been considered, and the others will hereafter be considered. They seem to me to par- take little of the character of demonstration, and to have little weight in comparison with those in favour of the power of natural selection, aided by the other agencies often speci- fied. I am bound to add, that some of the facts and argu- ments here used by me, have been advanced for the same purpose in an able article lately published in the 'Mcdico- Chirurgical Review.' At the present day almost all naturalists admit evolution under some form. Mr. Mivart believes that species change through "an internal force or tendency," about which it is not pretended that anything is known. That species have a I—HC XI