THE PROGRESS OF LIFE 99 



which it sprang. (2) The now dominant race, at the 

 beginning of its greatest vigour, gives origin to a new 

 type, which retires to some other place ; while the 

 future evolution of the dominant race is insignificant. 1 



These two hypotheses are different ; but they both 

 agree in thinking that there is some real resemblance 

 between the life of an individual, and the life of a 

 group ; that in both there is early vigour and sub- 

 sequent decline. But when we remember that all 

 living organisms are descended from those first formed 

 in the pre-Palaeozoic ocean, and that life on the earth 

 is, as a whole, quite as vigorous now as when first 

 produced, we see that the idea of the necessary 

 decline of a group in physiological vigour 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 excess 

 of the constructive (metabolic) processes : while the 

 opposite is the case in the growing tissue of the stem. 

 A change of leaves thus becomes necessary, but a 

 change of species is not necessary, unless something 

 alters the surrounding conditions. It is, however, 

 always the lower or unspecialised forms which give 



1 " Outlines of Vertebrate Palaeontology," Introduction, p. xxi. 

 (Cambridge, 1898). 



