CHARLES DARWIN. 123 



That the present lias been evolved from the past may be 

 shown and illustrated in various ways. The whole of living 

 nature may be likened to a tree. The root is as yet unknown, 

 but it is probably represented by some such simple form of life, 

 if life it be, as the Bathyhms of the ocean. The trunk soon 

 divides into two main branches, representing the vegetable and 

 the animal kingdoms, but, before it does so, forms are developed 

 which are iutfriiie;liate between plants and animals, or whicb at 

 one period of their life are animate, and at other periods possess 

 merely vegetative powers. Each main branch, the vegetal and 

 the animal, then ramifies ; the secondary branches, branchlets, 

 and twigs representing the sub-kingdoms, classes, orders, families, 

 genera, species, and varieties of our natural system of classification. 

 This system is natural, because, and only so far as, it is founded 

 on genetic relationship. The closer any two organisms agree 

 in structure, the nearer are they genetically related. But each 

 sub-kingdom is formed after a type which is followed with 

 modifications in every one of its ramifications up to the individual. 

 And just as certainly as each single leaf of a tree is vitally 

 connected with the root, has each individual plant and animal 

 been developed, by a purely generative process, in the course 

 of incalculable ages, from some simple or undifferentiated form 

 of living matter. In the utmost diversity there is unity. All 

 living things, from the lowliest plant to the highest animal, 

 have similar functions, they feed, grow, and reproduce their 

 kind ; protoplasm is in all the physical basis of life ; and all 

 forms of protoplasm are built up of the same elements — carbon, 

 hydi'ogen, oxygen, and nitrogen. A single origin for life alone 

 suffices adequately to explain this agreement. 



In animals, when typical structures are no longer useful, they 

 frequently remain, usually as rudimentary organs, the presence of 

 which can only be accounted for by genetic relationship and descent 

 with modification, for they are organs which have dwindled owing 

 to changed conditions rendering them useless. For instance, in 

 the course of adaptation of terrestrial quadrupeds to aquatic 

 habits, the hind-limbs dwindle. Thus, in the seals, the hind-legs, 

 although retaining all their typical bones, are almost rudimentary ; 

 and in the whales they are not apparent at all externally, and are 

 only represented internally by very rudimentary remnants. Again, 

 in the snakes there are no vestiges of fore-limbs, and only in the 

 python do we find vestiges of hind-limbs, as tiny rudiments under 

 the skin, and therefore quite useless to their possessor. These are 

 cases of degeneration of organs fi'om want of use, but they are 



