16 THE MAMMALIA [SECT. A 



while the second means of distinction (diet) would be called a 

 physiological criterion. Thus it is that we are compelled at the 

 outset to state clearly the basis upon which we proceed in a 

 classification. In the present case the basis is Morphology, and 

 our criterion is the criterion of structure. In this classification, 

 animals will be considered as more or less closely related, according 

 as their structure is more or less similar in its details. It is 

 necessary to enter into details for two reasons. First, because the 

 very fact of the name " vertebrate " being applicable to all the 

 animals about to be considered, implies that they possess one 

 general morphological character in common, and indeed that they 

 are constructed upon a similar plan. Clearly, therefore, we must 

 know more than this elementary fact before proceeding to any kind 

 of arrangement by groups. But in the second place we have to 

 remember that the principle of basing relationship on the ground 

 of similarity in structure is of fundamental importance. That the 

 principle itself may be overthrown one day is conceivable. 



That contingency is still sufficiently remote to leave an ample 

 margin of confidence in our guide, provided always that conclusions 

 are based on evidence drawn from a number of details and not from 

 single individual features or characters. This note of warning must 

 needs be sounded in view of the cases adduced as examples of 

 " Convergent Evolution." Without close attention to details, the 

 observer may well be deceived in such instances. Sometimes, as 

 for example where the fish is compared with the whale, the in- 

 vestigation need not proceed far before the real differences are 

 exposed. But again, when the marsupial mole (Notoryctes) and 

 the European mole are compared, a finer analysis is required. 



The problems encountered in Physical Anthropology involve 

 greater difficulties and a corresponding increase in the attention 

 due to details. And in the more particular consideration of the 

 principles of Morphology, the following point is of interest and 

 importance. It will be remembered that the vertebrate body 

 is composed of a number of very different parts, and that 

 although these are demonstrably similar to one another, in- 

 asmuch as they all possess cellular structure, yet the cells have 

 undergone specialization in several different directions, with the 

 consequent production of a number of tissues from which have 



