FUNDAMENTAL RELATIONS OF ANIMALS 67 



different and might be described as different species if seen isolated 

 or obtained from different regions, those of other species appear all 

 as cast in one and the same mould. It must be therefore at once ob- 

 vious how different the results of the comparison of one fauna with 

 another may be, if the species of one have been studied accurately 

 for a long period, by resident naturalists, and the other is known 

 only from specimens collected by chance travelers; or if the fossil 

 representatives of one period are compared with living animals, with- 

 out both faunas having first been revised according to the same stand- 

 ard.9^ 



Another deficiency in most works relating to the habits of animals 

 consists in the absence of general views and of comparisons. We do 

 not learn from them how far animals related by their structure are 

 similar in their habits, and how far these habits are the expression 

 of their structure. Every species is described as if it stood alone in 

 the world; its peculiarities are mostly exaggerated, as if to contrast 

 more forcibly with all others. Yet how interesting would be a com- 

 parative study of the mode of life of closely allied species; how in- 

 structive a picture might be drawn of the resemblance there is in 

 this respect between species of the same genus and of the same family. 

 The more I learn upon this subject, the more am I struck with the 

 similarity in the very movements, the general habits, and even in the 

 intonation of the voices of animals belonging to the same family; 

 that is to say, between animals agreeing in the main in form, size, 

 structure, and mode of development. A minute study of these habits, 

 of these movements, of the voice of animals cannot fail therefore to 

 throw additional light upon their natural affinities. 



While I thus acknowledge the great importance of such investiga- 

 tions with reference to the systematic arrangement of animals, I can- 

 not help regretting deeply that they are not more highly valued with 

 reference to the information they might secure respecting the ani- 

 mals themselves, independently of any system. How much is there 



"^ In this respect I would remark that most of the cases in which specific identity 

 has been affirmed between living and fossil species, or between the fossils of different 

 geological periods, belong to families which present either great similarity or extra- 

 ordinary variability, and in which the limits of species are therefore very difficult to 

 establish. Such cases should be altogether rejected in the investigation of general ques- 

 tions involving fundamental principles, as are untrustworthy observations always in 

 other departments of science. Compare further my paper upon the primitive diversity 

 and number of animals, quoted above, in which this point is specially considered. 



