200 



HABIT AND INTELLIGENCE. 



[chap. 



The dog 

 aud the 

 pigeon. 



The most 



constant 



characters 



the most 

 constant 

 in classes. 



many instances, especially in the races of tlie dog and of 

 the pigeon, which are so unlike each other that no one, on 

 merely seeing them, would guess that they belonged to the 

 same species ; and yet it is proved that they do belong to 

 the same species, in both cases, by the fact that they breed 

 freely together, and, in the case of the pigeon, by satis- 

 factory evidence that all the domestic races have originated 

 since the bird was first domesticated. If similar varieties 

 exist in the microscopic characters of any species, they will 

 not be obvious to the eye, but must be sought for. 



It is to be observed that those characters which are 

 most constant and least liable to variation, as between 

 in species individuals and between mere varieties of the same species, 

 appear, as a general rule, to be also the most constant 

 throughout whole wide classes. Thus, the structure of 

 nerve, muscle, and bone is much more constant between 

 species of the same order, and between orders of the same 

 class, than is the distribution of the nerves and the form 

 of the muscles and bones. To mention a very remarkable 

 instance : the skeleton of a pterodactyle, which was a 

 reptile organized for flight, has a strong resemblance in its 

 general outline to that of a bird ; but a microscopic exami- 

 nation of a fragment of one of its bones shows it to have 

 the structure belonging to the bone of a reptile.^ This law, 

 that the characters which are most constant throu<rhout 

 the species are also the most constant throughout the 

 group to which the species belongs, is, as I agree with 

 Darwin in thinking, of great importance in explaining the 

 origin of species. I only refer to it here as affording a 

 strong presumption in favour of the truth of the opinion 

 I have advanced, that the minutest structures are tlie lea&t 

 variable. 

 The lowest The lowest organisms, on the whole, are the most 

 are most"^ variable. Vegetables are, on the whole, much more lowly 

 vaiiable. organized than animals,, and they are much more variable 

 in form. The Algte, which are among the lowest vege- 

 tables, are peculiarly variable ; and among the Foramini- 

 • fera, which are among the lowest animals, the variability 



1 Carpenter's Comparative Physiology, p. 140. 



Bones of 

 pterodac- 

 tyle. 



