474 On Progressive Development, &c. (Serr. 
which here commences to display itself externally. From hence the 
gradual progress of the organ may be traced through the chirotes, the 
bipes and the seps. 
In the same way may be seen the gradual increase in the size of the 
left lung which had been retarded in growth in Ophidia—and the pro- 
gressive perfection of the organs of sense, of the osseous, and of the ner- 
vous systems. With regard to Chelonia, the highest in the class of cold- 
blooded vertebrata, the consideration of the numerous analogies which 
their anatomical structure shows to exist between them and warm- 
blooded animals, the commencement of a perfect division of the ven- 
tricle, and the evident transition from these animals to the class of 
birds, are subjects which would extend this paper beyond the limits of 
a brief memoir. 
In the endeavour to trace the connection between these different 
tribes of animals, itis to be remembered that the materials for investiga- 
tion are comparatively few ; that unacquainted as we are with the inter- 
nal structure, and more minute anatomical relations of the extinct 
races, we are deprived of the evidence most material to our cause ; yet 
imperfect as our knowledge of these animals must necessarily be, we 
are able to trace in their analogies with existing genera, a type inter- 
mediate between two important divisions of the animal kingdom, and 
occupying permanently the station now held temporarily by Batrachia 
during their metamorphosis. 
Examples of this kind, where the intermediate stages apparently 
wanting in our systems of zoology are to be discovered in the ancient 
strata of the earth, are very numerous. Among the fossil Echinoder- 
mata in the chalk formation, the gradation of development, from the 
flattened and ramified ewryale, through the clypeaster, the sentella, 
the ananchiie, the galerite and the spatangus, to the concentrated 
and spherical form of the cidaris and echinus, must strike the most 
cursory observer. The tertiary strata of the Paris basin, have furnish- 
ed us with the links which were wanting in the order Pachydermata to fill 
up the hiatus which separated the pig and the tapir from the ele- 
phant. 
If these observations be correct, no organ or system of organs, nor 
any new type in the animal world, can be said to have suddenly appear- 
ed on the stage of existence. There are certain laws to which nature 
herself is compelled to submit, and by which all her operations must be re. 
gulated ; and notwithstanding the weight which attaches to the opinions 
of the learned professor already quoted, I cannot help believing that 
amongst them is to be found the law of progressive development. 
