of certain Birds of Cuba. 21 
to vary much less than in Mammalia; and consequently, to 
merit in them more attention as a ground of division. Finally 
therefore, we may conclude, that in that order of Birds where 
this great principle of structure varies the most, there Nature 
is—if I may use the expression—looking out for the structure of 
some other class. 
This mode of reasoning is, I am fully aware, not without its 
defect ; but if it can be admitted to possess any value, it follows, 
that we must look among the Rasores and Natatores for the 
outlets from the class. And as there cannot be the least doubt 
of the Natatores* approaching to the Chelonian reptiles, we 
must consequently look among the Rasores for the approach to 
Mammalia. 
Now this agrees with theory, inasmuch as it is from the two 
extremes of the three Aberrant groups that we should expect to 
pass into the contiguous classes. 
But this question is of such extreme importance to zoology, 
that it ought not to be dismissed slightly. I trust, therefore, 
that I shall scarcely be deemed to trespass upon the time of the 
Society, if I here attempt to investigate three subjects, which 
have exercised the ingenuity of naturalists from the earliest 
periods of their science, and which are as follows: 
1. The true analogies existing between the orders of Birds 
and those of Mammalia. 
2. The connection of the various orders of Mammalia in their 
own series of affinityt. 
3. The point of nearest approach made by Mammalia to 
Birds, 
* Bonnet on this account divided the order into Aves Aquatica and Aves Amphibia. 
See on this subject Hore Entomologice, p. 263, et seq. 
+ I ought here to acknowledge, that the subject of the natural division of the Mam- 
matia has been taken up by a writer in the Avnals of Philosophy for November 1826. 
This gentleman not only shows much acquaintance with the class in detail, but has 
ingeniously developed a number of natural approximations. Unfortunately, however, 
first 
