MEANING OF ORDERS. 77 



Crab. While using illustrations of this kind, 

 however, I must guard against misinterpretation. 

 These embryological changes are never the pass- 

 ing of one kind of animal into another kind of 

 animal ; the Crab is none the less a Crab during 

 that period of its development in which it resem- 

 bles a Lobster ; it simply passes, in the natural 

 course of its growth, through a phase of ex- 

 istence which is permanent in the Lobster, but 

 transient in the Crab. Such facts should stimu- 

 late all our young students to embryological 

 investigation, as a most important branch of 

 study in the present state of our science. 



But while there is this structural gradation 

 among orders, establishing a relative rank be- 

 tween them, are classes and branches also linked 

 together as a connected chain ? That such a 

 chain exists throughout the Animal Kingdom 

 has long been a favorite idea, not only among 

 naturalists, but also in the popular mind. Lam- 

 arck was one of the greatest teachers of this 

 doctrine. He held, not only that branches and 

 classes were connected in a direct gradation, but 

 that within each class there was a regular series 

 of orders, families, genera, and species, forming 

 a continuous chain from the lowest animals to 

 the highest, and that the whole had been a grad- 

 ual development of higher out of lower forms. 

 I have already alluded to his division of the 



