438 PHYSIOLOGICAL ORIGIN OF SPECIES. 



in every stratum, for they give a facies to the fauna though the species 

 may be numerically few. 



Origin of Species. The philosophy of palaeontology was long re- 

 tarded by the views held concerning the nature of species, and although 

 it was considered that any number of varieties might be thrown off, 

 all the varieties were imagined to be infertile, or to revert in a state of 

 nature to the original type ; and it was not until the publication by Mr. 

 Charles Darwin, in 1859, of his work on " The Origin of Species," that 

 naturalists began to conceive of the possibility of species coming into 

 existence under laws which w^ere closely associated with their gradual 

 extinction. The two great principles which Mr. Darwin recognised 

 as aiding the development of life were " the struggle for existence," 

 and " the survival of the fittest," meaning by the struggle for exist- 

 ence the obvious fact that more individuals of all races are produced 

 than can survive ; and that those selected by Nature to survive are 

 the individuals or races which have physical or mental advantages 

 over the others ; and that, by continuous variation, a divergence might 

 in this way be produced which would result in the elaboration of new 

 species. The origin of species, however, is not a simple process, and 

 the cause of the structural variation is here left unexplained, or is 

 assumed to be a matter of accident. This logical defect in Mr. Dar- 

 win's original argument retarded for a time the acceptation of his views, 

 and led to a belief that the causes of evolution might admit of some 

 elaboration. We had arrived, in I862, 1 at the conclusion that, how- 

 ever important an agent in change the struggle for existence may be, 

 the fundamental active principle in evolution is physiological causation, 

 partly dependent upon external influences which govern the condi- 

 tions of food supply, but more dependent upon circumstances which 

 govern digestion, nutrition, and elimination of waste products and 

 unused structures ; and we still believe that too much attention cannot 

 be given to the physiological aspects of evolution. 



Thus, where animals are fixed on the sea-bed, and derive their 

 food supplies from currents, or where animals are dependent on the 

 vibratile motions of cilise, it is difficult always to realise the condi- 

 tions of a struggle for existence, or the nature of endowments which 

 would characterise a favoured race. But from the physiological point 

 of view the problem is simpler, for all variation is ultimately a matter 

 of organic dialysis, in which the action of the dialysing membrane of 

 a tissue or organ varies with its own nutrition and physical conditions, 

 and hence elaborates or eliminates more or less of a tissue, or modifies 

 the tissue so as to vary the sum of the animal structures under 

 the conditions of geological change. The principle which would 

 account for the origin of species must be equally applicable to all 

 types of life ; and because there is nothing in common between all 

 types of life except the modes of nutrition of the tissues, the func- 



1 Camb. Phil. Soc., March 17, 1862, "Researches on the Homologies of the 

 Bivalve Mollusca, and therein of the Law of the Variation of Forms and the 

 Nature of Genera." This memoir was only printed in abstract in the Cambridge 

 Chronicle. 



