Embryology. 103 



a more general'view of the whole subject, I will begin 

 at the foundation, and gradually work up from the 

 earliest stages of development to the latest. Before 

 starting, however, I ask the reader to bear in mind 

 one consideration, which must reasonably prevent 

 our anticipating that in every case the life-history of 

 an individual organism should present a full recapitu- 

 lation of the life-history of its ancestral line of species. 

 Supposing the theory of evolution to be true, it must 

 follow that in many cases it would have been more or 

 less disadvantageous to a developing type that it 

 should have been obliged to reproduce in its individual 

 representatives all the phases of development pre- 

 viously undergone by its ancestry even within the 

 limits of the same family. We can easily understand, 

 for example, that the waste of material required for 

 building up the useless gills of the embryonic sala- 

 manders is a waste which, sooner or later, is likely to 

 be done away with ; so that the fact of its occurring 

 at all is in itself enough to show that the change from 

 aquatic to terrestrial habits on the part of this species 

 must have been one of comparatively recent occurrence. 

 Now, in as far as it is detrimental to a developing 

 type that it should pass through any particular ances- 

 tral phases of development, we may be sure that natural 

 selection or whatever other adjustive causes we may 

 suppose to have been at work in the adaptation of 

 organisms to their surroundings will constantly seek 

 to get rid of this necessity, with the result, when 

 successful, of dropping out the detrimental phases. 

 Thus the foreshortening of developmental history 

 which takes place in the individual lifetime may be 

 expected often to take place, not only in the way of 



