266 EVOLUTION, OLD AND NEW. 



even more important for classifications than that of the 

 adult." * 



In his second chapter Lamarck deals with the im- 

 portance of comparative anatomy, and the study of 

 homologous structures. These indicate a sort of blood 

 relationship between the individuals in which they are 

 found, and are our safest guide to any natural system 

 of classification. Their importance is not confined to 

 the study of classes, families, or even species; they 

 must be studied also in the individuals of each species, 

 as it is thus only, that we can recognize either identity 

 or difference of species. The results arrived at, how- 

 ever, are only trustworthy over a limited period, for 

 though the individuals of any species commonly so 

 resemble one another at any given time, as to enable 

 us to generalize from them, at the date of our observing 

 them, yet species are not fixed and immutable through 

 all time : they change, though with such extreme slow- 

 ness that we do not observe their doing so, and when 

 we come upon a species that lias changed, we consider 

 it as a new one, and as having always been such as we 

 now see it. t 



" It is none the less true that when we compare the 

 same kind of organs in different individuals, we can 

 quickly and easily tell whether they are very like each 

 other or not, and hence, whether the animals or plants 

 in which they are found, should be set down as 

 members of the same or of a different species. It is 

 only therefore the general inference drawn from the 



* ' Origin of Species,' p. 395, ed. 1876. 

 t k Phil. Zool.,' torn. i. p. 61. 



