12 MORPHOLOGICAL DEVELOPMENT. 



metamorphoses. Throughout the animal world these are 

 variously displayed by parasites, multitudinous in their 

 kinds; for most of them belong to types much higher in 

 organization. Changed habits and consequent changed struc- 

 tures have so transferred them that only by study of their 

 embryonic stages can their kinships be made out. And these 

 retrograde metamorphoses, conspicuous among parasites, have, 

 in the course of evolution, affected some members of all 

 groups; for in all groups the struggle for existence has com- 

 pelled some to adopt careers less trying but less profitable. 



Not only by forcing on many kinds of organisms simpler 

 ways of living, and consequent degeneracy, has the universal 

 competition caused obscuring transformations. It has done 

 this also by tempting many other kinds of organisms 

 to adopt ways of life not simpler than before but merely 

 different. Pressure continually prompts every type to in- 

 trude on other types' spheres of activity; and so causes it to 

 assume certain structural characters of the types whose 

 spheres it invades, masking its previous characters. Modifi- 

 cations hence arising have, in the great mass of cases, been 

 superposed one on another time after time. The aquatic 

 animal becomes through several transitions a land-animal, 

 and then the land-animal through other transitions becomes 

 now an aerial animal like the bat and now an aquatic animal 

 like the whale. Certain kinds of birds furnish extreme 

 illustrations. There was the change from the fish to the 

 water-breathing amphibian and then to the air-breathing 

 amphibian; thence to the reptile living on the Earth's sur- 

 face; thence to the flying reptile and the bird; then came 

 the diving birds, joining with their aerial life a life passed 

 partly in the water; and finally came a type like the 

 penguin, in which the power of flight has been lost and the 

 water has again become the almost exclusive medium, except 

 for breathing. Of course the mouldings and re-mouldings of 

 structure resulting from these successive unlike modes of 

 life, in many cases put great difficulties in the way of ascer- 



