RETROGRESSION IN THE EMBRYO 121 



development in the higher animals. In them is achieved the same 

 end the fore-shortening of long and changeful periods of the life- 

 history. All, or almost all rapid recapitulation occurs during 

 periods of quiescence when the individual is protected or con- 

 cealed, that is when he is least exposed to selection. Better than 

 anything else development indicates the extent to which the 

 organism is a bundle of adaptations. Thus the embryo of the 

 butterfly develops fast while it is stationary in its egg-case, where 

 it remains till food is abundant ; the active caterpillar, fitted to its 

 environment by all its structures and instincts, stores nutriment in 

 its tissues against a time when, did it continue active, it would be 

 very disadvantageously situated while changing into the adult 

 insect. The human infant is born to a mother who is fitted to tend 

 just such a developing being at just such a stage of its develop- 

 ment. Were its development different, did it recapitulate the life- 

 history in fuller detail, she could neither bear nor tend it. In both 

 the caterpillar and the human being the abbreviations of the life- 

 history, though happening at different periods of life, occur at 

 precisely the most useful periods. After the period of reproduc- 

 tion of offspring the fitness of the individual to the environment 

 declines swiftly in the lower animals, more slowly in animals that 

 tend their young. For obvious reasons, Natural Selection can no 

 longer preserve it. There is no further recapitulation. There is 

 nothing to recapitulate. 



200. Plants lead a life more quiescent than most animals. 

 Speaking generally, the seed drops to the ground, and the young 

 plant emerges from it into an environment which hardly changes 

 again during the lifetime of the individual, but which, since the higher 

 plants have evolved from lower forms, must have changed greatly 

 for the species. In them, therefore, retrogression has so abbreviated 

 a major portion of the life-history that no one merely examining 

 the embryo of an oak, for example, would gather more than a hint 

 of the immensely long and changeful process by which the species 

 underwent evolution. 



201. We see then how great a part retrogression plays in 

 evolution and by what varied devices nature assists its operations. 

 It swiftly eliminates the useless progressive variations, the redund- 

 ancies, which occur in vast numbers in every individual. More 

 slowly, in proportion to their antiquity, it rids the species of 

 characters which have lost their utility. It abbreviates and 

 simplifies development. It wars against progression, but great 

 progression would be impossible without it. It furnishes a half of 



