256 



HABIT AND INTELLIGENCE. 



[chap. 



when the original shapeless germinal mass begins to 

 assume the form of the species, it first assumes an em- 

 bryonic form, which is common to all organisms that are 

 constructed on the same general plan, whether that be 

 vertebrate, or articulate, or any other : it afterwards 

 assumes the definite form of its species ; and the high- 

 est species, being the most differentiated, depart most 

 widely from the common embryonic form. Thus in- 

 sects, for instance, being higher than centipedes or 

 worms, depart more widely than do centipedes or worms 

 from the embryonic or larval form which is common 

 to all. 



If the development theory of the origin of species is 



true, there ought to be a perfect series of species present- 



incr transitional forms between those lowest ones which 



depart least in the course of their individual development 



from the common embryonic form of the gToup, to those 



highest ones which depart from it most widely. Between 



worms and insects, most of the transitional forms appear to 



be lost ; but between fishes and the highest Batrachians 



there is an almost unbroken series of intermediate forms. 



Transition This series begins with the lepidosiren, an animal which 



^"^ tlr*^"^ is classed by Owen as a Batrachian, but by some naturalists 



and air- as a fish. The series is continued through the Perenni- 



Verte-"^^ branchiate Batrachians, a remarkable transitional order 



brates. which have two sets of respiratory organs : branchiae like 



those of the fish for breathing Avater, and lungs like those 



of the higher Vertebrata for breathing air. Next in the 



series come the newts, toads, and frogs, which differ from 



the Perennibranchiates in losing their branchiae when their 



development is completed, so as to become air-breathers 



exclusively. And lastly comes the Sakmiandra atra, or 



land salamander, which, like the higher Vertebrata, passes 



through its metamorphoses in the egg, and leaves the egg 



in the air-breathing form. 



It is most important to observe, that there is a close 

 parallelism between the series of specific forms in the 

 Pereunibranchiate order, and the series of forms through 

 which each individual among the higher or exclusively 



