76 EXPERIMENTAL EVOLUTION LECT. 



artificial race at its highest standard, and we all know 

 how readily degeneracy interferes with and ruins the 

 work of man or nature. 



Dimensional variations, although very considerable, 

 cannot be regarded as unlimited. We cannot expect 

 to make any species of plant or animal become much 

 larger or much smaller than it is. Of course there are 

 natural or artificial conditions Under which all species 

 acquire a better development, and many facts display 

 this. But we cannot expect to be able to increase 

 the dimensions of any species beyond a certain point. 

 Such an increase would require numerous variations in 

 all the systems of the organism, stronger bones for 

 instance, a stronger heart, and so on. 1 And then, on 

 another side, giant forms would require so much food 

 that their number could never become very large, and 

 in fact, much goes to prove that such forms would 

 have much trouble to compete with others, while the 

 smaller forms could more easily live and maintain 

 themselves. So there certainly exists a limit to the 

 increase of dimensions a physiological limit which 

 cannot be passed without danger to the organism. 

 Conversely, there is also a limit to the decrease of 

 dimensions. Too small animals or plants are too 



1 Paul Bert (Sur le Maximum de Tailk que puissent atteindre les 

 Animaux Vertebres : Soc. de Biologic, 1878) considers the maximal 

 dimensions of vertebrate animals as dependent upon the strength of the 

 cardiac muscle. 



