Conclusion 143 



tinually have to expend energy to maintain the osmotic equilibrium 

 of their internal fluids. Locomotion has become swifter, better con- 

 trolled, and more purposeful. The nervous system has acquired 

 better correlating mechanisms and greater centralization of control. 

 The sense organs have become better adapted for distance percep- 

 tion, and less dependence is placed on contact sensations in making 

 rapid adjustments to changes in environment. Excretory organs 

 which perhaps began as regulators of osmotic pressure have taken 

 over the elimination of metabolic waste products, especially those 

 containing nitrogen. Reproductive functions have been improved 

 and adapted by internal fertilization; protection of young from 

 thermal changes, desiccation, and other dangers; and by the nour- 

 ishment, care, and education of the young. Adaptations have been 

 developed for the utilization of the great food resources in land 

 habitats; especially the products of hard, but concentrated and nu- 

 tritious, vascular plants. Through slow acclimatization to new 

 conditions animals have changed from sluggish systems of activities 

 in a uniform environment where little energy is available to active, 

 speedy systems. Land animals by keeping their internal mechanisms 

 uniform and by moving at a rapid metabolic rate are able to live 

 successfully in a variable environment where much energy is avail- 

 able. 



Fig. 23. A sandy shore at Portsmouth in Dominica. 

 Home of the ghost crab and other burrowers which 

 are gradually emigrating toward land. (Photo by 

 R. E. Blackwelder.) 



