INTRODUCTORY OBSERVATIONS. 87 
anatomical details of the steps by which these changes take 
place, it is not my intention to enter; but the general de- 
velopment of the new organs, both of these functions and 
of that of progression, will be given in the account of each 
different form. 
We find then that these typical forms of the Amphibia 
become essentially altered during the progress of their 
growth in all their principal systems of organization ; in 
the nervous, the circulating, the respiratory, the digestive, 
and the reproductive organs: nor does reproduction ever 
take place in these animals until the other changes have 
been perfected. But in the perennibranchiate forms, as the 
Siren, the Proteus, &c. it seems as if the metamorphosis 
were stopped suddenly at that period when the lungs begin 
to be developed, before the branchiz have at all diminished 
in size or in activity of function. The reproductive organs, 
however, go on to their full development, and the animals 
never undergo any further change of form or habit, but 
continue throughout life to breathe both the atmosphere 
by their air-cells, and water by their branchiz, as well as 
either medium indifferently by means of their skin. In a 
word, the pulmonary, the branchial, and the cutaneous 
modes of respiration are in these curious animals going on 
simultaneously, although there can be no doubt that the 
branchial is the most essential to their well-being, and the 
pulmonary the least so. 
Enough has probably been said on the general physio- 
logy of these animals; for it would not consist with the 
object of this work to enter more minutely into the details 
of this part of the subject; but to the physiological in- 
quirer, few classes of animals present a more extensive or 
interesting field of investigation. After all that has been 
done, much remains yet to be ascertained, in the functions 
