AND THE LOWER ANIMALS. 41 



The branchial respiration and its consequent peculiar 

 circulation are followed by pulmonary respiration, and 

 the true and permanent circulation of the blood. In- 

 stead of living on the juices of the mother, the animal 

 now digests its own food. Thus in a few days takes 

 place the last important transformation which a mam- 

 mal undergoes ; and since this converts the being from 

 a parasite into an independent animal, we may fairly 

 term it a metamorphosis . 



The phenomena we have been describing, no 

 matter what be their complexities or rapidity, succeed 

 each other in regular order in each species of mammal, 

 when the development is systematic ; but they are 

 occasionally interrupted by various disturbing causes, 

 some of which we suspect, others we are entirely un- 

 acquainted with. Organs may have their transforma- 

 tions interrupted without the vital force being arrested, 

 or the growth of the new creature checked ; but then 

 these organs depart from the natural type or plan. It 

 is thus that monstrosities are produced. We see, 

 therefore, that these departures from the normal plan 

 take place at a very remote period of embryonic life, 

 and that, cceteris paribus , the earlier the date of the 

 disturbing cause, the more extensive will these depar- 

 tures be. Hence M. I. Geoffroy had good reasons for 

 asserting that all mammalian monstrosities are con- 

 genital, or, in other words, that they take place prior 

 to the birth of the animal. ; We may say, then, that all 

 monstrosities are produced by an accidental, but deci- 

 dedly embryogenical phenomenon. Having stated this 

 general law, whose importance will be explained by 

 the study of animals which undergo metamorphoses, 

 we shall return to mammalia. 



