THE SUCCESSION OF ANIMAL FORMS 195 



their younger stages, and rapidly assume all their varietal forms, 

 and extend themselves as widely as external circumstances will 

 permit. Like individuals also, they have their periods of old 

 age and decay, though the life of some species has been of 

 enormous duration in comparison with that of others; the 

 difference appearing to be connected with degrees of adaptation 

 to different conditions of life. 



5. Many allied species, constituting groups of animals and 

 plants, have made their appearance at once in various parts of 

 the earth, and these groups have obeyed the same laws with 

 the individual and the species in culminating rapidly, and then 

 slowly diminishing, though a large group once introduced has 

 rarely disappeared altogether. 



6. Groups of species, as genera and orders, do not usually 

 begin with their highest or lowest forms, but with intermediate 

 and generalized types, and they show a capacity for both eleva- 

 tion and degradation in their subsequent history. 



7. The history of life presents a progress from the lower to 

 the higher, and from the simpler to the more complex, and 

 from the more generalized to the more specialized. In this 

 progress new types are introduced, and take the place of the 

 older ones, which sink to a relatively subordinate place, and 

 become thus degraded. But the physical and organic changes 

 have been so correlated and adjusted that life has not only 

 always maintained its existence, but has been enabled to 

 assume more complex forms, and thus older forms have been 

 made to prepare the way for newer, so that there has been, on 

 the whole, a steady elevation culminating in man himself. 

 Elevation and specialization have, however, been secured at the 

 expense of vital energy and range of adaptation, until the new 

 element of a rational and inventive nature was introduced only 

 in the case of man. 



8. In regard to the larger and more distinct types, we 

 cannot find evidence that they have, in their introduction, 



