256 PROCEEDINGS OF THE ACADEMY OF [1895. 



destructive to the lower animals than to Man, the difference being 

 due to difference of life habits. Though bacteria and other disease- 

 producing agents may at times in the past have attacked species of 

 animals destructively, it is probable that they have played but a 

 minor part in the extinction of species. 



There is still another interesting natural condition to consider in 

 our review of the general causes of the extinction of species. One 

 tendency, which has particularly manifested itself in herbivorous 

 animals, has frequently led directly to their destruction. This is the 

 tendency to increase in size through the double influence of abund- 

 ance of food and little waste of tissue through exertion. In the 

 sluggish grass-eaters, dwelling on plains covered with rich herbage, 

 or leaf and twig eaters in tropical forests, the nutritive agencies are 

 in excess of those of waste, and these animals seem always to have 

 tended to an increase in size, until those of least exertion and 

 greatesl powers of obtaining food became enormous in dimensions. 

 An example of the same kind among the carnivora is the Green- 

 land whale, which, while feeding on minute forms, obtains them in 

 enormous quantities with little muscular exertion, and has in conse- 

 quence become of extraordinary dimensions. 



In the case of the herbivora this increase in size has exposed them 

 to increasing danger of starvation in cases of great drought, and from 

 the food competition of smaller but more numerous animals, and 

 many species may have became extinct through this cause. It is 

 probable that a struggle has long gone on between the two organic 

 tendencies — on the one hand to increase in bulk; on the other to 

 increase in activity — the victory finally falling to the smaller, more 

 active, and more mentally energetic forms, through their ability to 

 survive on less food and their superior powers of resistance to nature's 

 adverse influences. ft is perhaps mainly due to this that the bulky, 

 sluggish and mentally dull creatures of the past have given way to 

 the smaller but more active and intelligent animals of the present. 



This leads us directly to the problem of the disappearance of the 

 great Cretaceous reptiles — the first of the two special cases to be con- 

 sidered. The influences described may have had something to do with 

 this event, but are far from sufficient to explain the sudden dis- 

 appearance of so many species of animals of varied habitat, food and 

 conditions of life. However far land animals may have been 

 thus affected, the great ocean reptiles could hardly have succumbed 



