2 S 6 



CHAPTERS ON EVOLUTION. 



development than a small egg, in which no provision exists for the 

 nutrition of the embryo. So much, indeed, may safely be predicted 

 of the causes which retard or favour an early escape from the egg. 

 In the latter case, of course, let us bear in mind that the young will not 

 resemble the parent animal, and we naturally expect to behold 

 changes of form or " metamorphosis " in its further development, and 

 ere it attains to the parent size and likeness. 



But we must not neglect to note an equally important cause of 

 alteration in form, which, acting subsequently to the escape of the 

 immature animal from the egg, will direct its footsteps in different 

 channels, and clothe its form with varied guises. The surroundings 

 of an animal's life necessarily affect that animal, and in time its 

 race, viewing individual and race as consisting each of an adult 

 being and beings. This much is the plainest of plain truths. 

 But it is equally true that surroundings and varying conditions 



FIG. 170. THE ROSY FEATHER-STAR'S DEVELOPMENT. 

 a, adult starfish ; b, young stalked forms. 



of life must also affect the young stages of animal existence. 

 Even more marked and powerful imist be the effect of outward 

 conditions on the young organism, whose frame and constitution, 

 not yet fully formed, are infinitely more plastic and facile than those 

 of the adult. All we know of the effects of environments on living 

 beings, teaches us this lesson. We know something of the effects of 

 heat and cold, of a change of medium, and of numerous other 

 circumstances which materially alter the development of both animals 

 and plants. Natural- history records teem with examples of these 

 facts. A young rosy feather-star (Antedon, Fig. 170) may be hurried 

 through its larval state, and may be made to gallop post-haste through 

 its " metamorphosis," if it be supplied with pure sea water. If, on 

 the other hand, such a larva be kept at a low temperature, and in 

 water not frequently changed, and consequently on a more meagre 

 dietary, it will delay in its larval progress. Its development may 



