THE THEORY OF NATURAL SELECTION. 301 



opinions. Mr Darwin himself believed that it was at 

 any rate ' the most important ' means. Whatever may be 

 the view ultimately adopted as regards this point, there is 

 overwhelming evidence in favour of the belief in some 

 general law of evolution, by which all animal and vegetable 

 species have been produced. The evidence in favour of 

 this may be briefly stated as follows : 



(1) All the animals belonging to each great primary 

 division of the Animal Kingdom are constructed upon one 

 fundamental plan, which is capable of endless modifica- 

 tions, but is never lost. Thus, to give one example, the 

 fishes, amphibians, reptiles, birds, and quadrupeds, which 

 together constitute the ' sub-kingdom ' of the Vertebrate 

 Animals, are all built according to one common plan. 

 However unlike they may be to one another in the details 

 of their organisation, ' homologous' structures can be 

 traced throughout the ground-plan of them all. This 

 unity of plan in the types of life which compose each 

 sub-kingdom is, however, inexplicable upon any other 

 view than that it is the result of blood-relationship, 

 and depends upon descent from a common ancestor, 

 which possessed the essential structural characters dis- 

 tinctive of Vertebrates as a whole. 



( 2 ) The animals composing each sub-kingdom are con- 

 structed upon the same plan, and the 'sub-kingdoms,' 

 taken as whole, stand therefore separate and apart. 

 But there exist transitional forms by which one ' sub- 

 kingdom is linked with another. Thus the singular 

 marine animals known as the Sea-squirts (Tunicata} form 

 a link between the true Shell-fish (MoUitsca) and the 

 Vertebrate Animals. In certain points, namely, in their 



