5 iv.] The question of species. 25 



total of the separate individuals and generations which, during 

 the time afforded for observation, exhibit the same periodically 

 repeated course of development within certain empirically deter- 

 mined limits of variation. We judge of the course of develop- 

 ment by the forms which make their appearance in it one after 

 another. These are the marks by which we recognise and dis- 

 tinguish species. In the higher plants and animals we are 

 in the habit of taking the marks chiefly from a single section of 

 the development, namely, from the one in which they are most 

 distinctly shown. We distinguish birds better by their feathers 

 than, for instance, by their eggs. This abbreviated method of 

 distinguishing is convenient, wherever one section of the de- 

 velopment is so pregnant as to make the consideration of the 

 rest unnecessary. But this is not always the case. The simpler 

 the forms of an organism are, the larger must be the portions 

 of development requisite for characterising and distinguishing it, 

 and the demand is still greater when we have to compare the 

 entire course of the development of the species, to use the same 

 figure, from the ovum of the first to the ovum of the next gene- 

 ration. We are pleased if we succeed in this way in finding any 

 single mark to serve our purpose, but we must not be too con- 

 fident of finding one. 



Experience has taught us that different species may behave 

 very differently in respect of the forms which make their appear- 

 ance successively in their course of development. In some the 

 same forms constantly recur one after another with comparatively 

 small individual variations. These may be termed monomorphic 

 species. Most of the common higher plants and animals are 

 examples of this, and also many of the lower and simpler kinds. 

 They can be readily distinguished after a little experience even 

 by single portions from the general development. We can re- 

 cognise a horse-chestnut, for example, by each individual leaf 

 plucked from the tree. 



Other species are pleomorphic and may appear in very 

 unlike forms even in the same segments of the development, 



