100 EVOLUTION THE RESULT OF CHEMICAL FORCES. 



bility without the valuable services of this fleetest and hardiest 

 of quadrupeds, nature's common-carrier. 



When we look back over the various geological eras, during 

 which the earth was under the reign of different orders of the 

 animal creation, we may very pertinently ask the question, why 

 these were not each in its time sufficient for all the requirements 

 of the great laws of food supply and the restriction of over- 

 production ? The races of the saurian reptiles ate up and killed 

 off the redundancies of population as effectually as ever later 

 races did it. The marsupial families that succeeded them were 

 abundantly able to use up the food supply of their age, and to 

 keep up the great balance of the producing and destroying forces 

 of nature. But these again proved to be on an unsuccessful pat- 

 tern, and they passed away, giving place to the quadrupeds of 

 more recent times. These possessed the earth and fully filled their 

 spheres until the tribes of mankind appeared, which have killed 

 off and wasted the natural products until pretty nearly all balances 

 are upset and there is but little of real nature left. Now what 

 was there in the requirements of the ages or the laws of natural 

 supply and demand that called into being the low r ly marsupials 

 to take the place of the gigantic reptiles, or the more delicate 

 placentals to supercede the pouched animals, or finally the bipeds 

 to exterminate the wild races of quadrupeds? It seems to me 

 that nothing short of an inherent and independent principle of 

 advancement in the races themselves can explain these anomalies 

 of development. 



It will be more f ullj r shown hereafter that the great physiolog- 

 ical changes occurring in the transitions from order to order and 

 family to family of the living kingdoms, could not, in very many 

 cases, have had intermediate stages, from the fact that anything 

 less than the full and complete change would have been, not only 

 of no advantage, but often a positive detriment or cause of 

 destruction to the organisms. In these cases, as we might ex- 

 pect, no connecting forms have ever been found. It is as if 

 nature had made a leap from one grade to another. I can explain 

 in no other way the following instances of great and apparently 

 sudden development. One is the transition from endogenous 



