- MEANING OF ORDERS. 7 
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 ina 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 
