NOTES. 



INTEODUOTION. 



Note \,page 10. Professor Marsh has lately discovered a colossal fossil 

 reptile, belonging to the family of the AtlantoBauridffi, and which was 

 of such enormous size — more than eighty feet long — that it could hardly 

 have been able to drag Itself along on the ground or even to raise itself, 

 if its bones had been as heavy in proportion as those of the reptiles or 

 mammals now extant. And in fact the bones of this fossil Saurian are 

 remarkably light in comparison with their size, so that Professor 

 Marsh's view that the peculiar large cavities which occur in all the 

 bones of the skeleton were air-cavities, and Ihat they were thus 

 aobually pneumatic bones, seems highly plausible. 



Note 2, page 17. There must, of course, be a number of characters 

 of adaptation, of which we cannot avail ourselves as hereditary cha- 

 racters of general importance, from their appearing only in small groups, 

 or merely in genera and species, or even in single individuals ; these 

 lose all diagnostic value in an inquiry as to the relations of affinity 

 among the larger categories to which the animals under comparison 

 may belong. Hence arises the necessity, insisted on in the text, for 

 inquiring how far hereditary characters of general value are to be dis- 

 tinguished from characters of adaptation. But every character which 

 can be regarded as a true sign of the common descent of large groups of 

 animal forms, may be ultimately traced to the stage at which it first 

 appeared, and where it was a character of adaptation. An example 

 will help to explain this. We know that in all the Vertebrata, without 

 exception, the first appearance of the skeleton, which is the most im- 

 portant organic system of the vertebrata, is inseparable from the 

 presence of an axial cord, known as the notochord, or chorda dorsalis. 

 This does not become merged in the vertebral column, but is displaced 

 by it ; it is perfectly inarticulated — a simple string of cells. The great 

 uniformity of the conditions of its first appearance, structure, and position 



