PALAEONTOLOGY 85 



must be a mistake. The decline is in numbers, and 

 not in the vigour of each individual. The decline is 

 due, not to any natural exhaustion, for it does not 

 always take place, but to the action of natural 

 selection. 



A closer analogy might be made between species 

 and the leaves of trees, which die off while the stem 

 remains alive. But this also, if pushed too far, would 

 be misleading ; for the leaves do not nourish them- 

 selves directly, but only the main body of the tree; 

 and they become exhausted and die off, because in 

 them the destructive (catabolic) processes are in ex- 

 cess of the constructive (metabolic) processes : while 

 the opposite is the case of the growing tissue of the 

 stem. A change of leaves thus becomes necessary, 

 unless something alters the surrounding conditions. 

 It is, however, always the lower or unspecialised 

 forms, which give rise to new groups ; and these un- 

 specialised forms are usually small. The specialised 

 forms, which answer to the leaves in the analogy, 

 die out, because, when a change in their habits 

 becomes a necessity, they cannot change their bodily 

 shape. This is not a proof of degeneracy, or of 

 decline in vigour. When man takes these highly 

 specialised plants or animals in hand, and domesti- 

 cates them, they shew no sign of loss of vigour ; and 

 in time they regain the power of variation. 



It is generally acknowledged that the sudden 

 appearance of a new group, in large numbers and in 

 considerable variety, is due to migration; and this 

 implies that the group was developed in some district 

 which is now either inaccessible or unexplored. 



