INTRODUCTION. 5 



the digits may be perpetuated amongst descendants 

 for a considerable time. Such cases have a parallel- 

 ism with the loss of certain vertebree in tame pigeons, 

 as noticed above. Professor Huxley again says, " the 

 word ' species ' has a double sense, and denotes two very 

 different orders of relations. When we call a group 

 of animals or plants a species, we may imply thereby 

 either that all these animals or plants have some 

 common peculiarity of form or structure, or may mean 

 that they possess some common functional character. 

 That part of biological science which deals with form 

 and structure is called morphology, that which con- 

 cerns itself with function physiology, so that we may 

 conveniently speak of those two senses or aspects of 

 species. Once more, " regarded from the former point 

 of view, a species is nothing more than a kind of animal 

 or plant which is distinctly definable from all others 

 by certain constant, and not merely sexual, morpho- 

 logical peculiarities."* 



De Candolle says a species is a collection of all the 

 individuals which resemble each other more than they 

 resemble anything else, which can by mutual fecunda- 

 tion produce fertile individuals, and in such a manner 

 that we may suppose them sprung from a single indi- 

 vidual. 



The characters of a species to be complete should 

 include all its forms, perfect and imperfect, modified 

 and unmodified ; since in this way only can their 

 " capacity for variation " be determined. f 



If we exhaustively carry out the modern (ancient ?) 

 doctrine of evolution into transmutation, we would 

 appear in great measure to eliminate all ideas of con- 

 stancy or permanence of species, for here persistent 

 varieties are conceived to arise from the stimulus of a 

 suitable environment, if only the operating conditions be 

 spread over a sufficient lapse of time. The explanation 

 of this change is surely only eluded by the introduction 



* Huxley's ' Lay Sermons,' p. 283—289. 



f Dr. W. B. Carpenter, ' Elements of Physiology.' 



