386 The Morality of Nature 



It seems probable that a distinct impulse under new en- 

 vironment, giving great new advantage, develops a general 

 plan of structure quickly, and then submits it to long con- 

 tinuing modifications by which adaptation to environment is 

 preserved and increased. But the discontinuous character 

 of the chain of related forms, shows that many such results 

 were of transient value ; and the separate genealogies of the 

 great divisions of animals, clearly show the existence of sev- 

 eral plans of structure of almost unrelated origin. 



There are, moreover, in the embryology of animals, reve- 

 lations of origin, which would never be suspected from the 

 present status of the mature animal. It appears clearly in 

 these cases that evolution upon a certain plan, has proceeded 

 to a certain position, and then has changed completely. 

 Sometimes progress toward a high type is revealed in the 

 early development, only to be succeeded by a backsliding 

 into low conditions; as for example in the Tunicata which 

 begin life much as do the higher animals ; and develop organs 

 which later degenerate and disappear. Then again em- 

 bryology discovers developments which follow a certain 

 plan, in symmetry of a certain kind, and then abandon that 

 form, for a later evolution of an entirely different symmetry, 

 reaching a high grade of structure by giving up the one first 

 begun and taking another. There are many well known 

 examples of larval life in which a creature, perhaps fish- 

 like and water breathing, abandons organs well developed, 

 and begins anew the structural work found wanting in new 

 circumstances. These facts show that there is no privilege 

 of type, even in high evolution, except by continued efficiency 

 and fitness, and that no type or order of structure is perfect 



