90 PROBLEMS OF EVOLUTION 



recapitulate the history of the race, and that many chapters are 

 missing. But it is most essential to the species, which, after 

 all, is more important than the evolutionist, that the ontogeny 

 should be brief. When once a higher level has been attained 

 by the species, the individual organism cannot afford to rest long 

 on the base degrees by which the ascent was made. Still less 

 can it afford to carry about with it all the primitive machinery 

 of the past. How would it be for man if, in spite of the 

 acquisition of lungs, he retained the gills by which his pro- 

 genitors breathed long ago ? In any organism there is good 

 reason to believe that all the important features are adaptations : 

 they are part of the equipment by which it maintains itself in 

 a world of combat and competition. And the fact that adapta- 

 tion is the rule is a proof that there has been a perpetual 

 shedding of machinery for which other and better has been 

 substituted. 



While, however, it is easy to realise the importance of the 

 general principle, it is not always easy to explain the process. 

 We may distinguish three cases: (l) Where the structures lost 

 are harmful, in that they render the organism liable to capture, 

 wounds or disease ; (2) where they are harmful, because their 

 maintenance taxes the vital force ; (3) where, though superfluous, 

 they are not injurious. 



Retrogres- In the first case the loss admits of a very simple explanation : 

 "i Natural structures tnat are injurious will be weeded out by Natural 

 Selection Selection. But simple as the explanation is, I shall quote one 

 or two familiar examples to show what an important part retro- 

 gression plays in evolution. Many of the crustaceans pass 

 through a free swimming Nauplius stage, the normal sequel to 

 which is some such form as the shrimp or water-flea. But 

 where the species has adopted a sedentary form of life, all the 

 promise of the embryo is falsified. 



Sacculina is an extreme instance. It is in appearance a huge 

 tumour on the abdomen of its host, the hermit crab. It sends 

 out a number of filaments which penetrate the whole body of its 

 victim and extract nourishment from it. So completely lost is 

 the crustacean form that there is no trace of segmentation, no 



