INSESSORIAL BIRDS. 239 



we met with the remains of a species in deposits of as old a date as 

 any in which birds were known to exist, Ave might be the less sur- 

 prised if that species exhibited a lower grade of development thaa 

 living species, and required to be so placed as to direct attention to 

 that fact ; but such a circumstance would cause no more difficulty 

 under our system than under any other, and it is evident that Mr. 

 Wallace has been reasoning from a misconception, so far at least as 

 any system of definite numbers is at present maintained. 



The law we have proposed may be well defended simply as the 

 general expression of a sufficient; number of observed facts, but its 

 interest and value are greatly increased if we are able to trace it ta 

 a general principle, and show a connection between it and great 

 truths respecting the structure of the animal kingdom. The living 

 functions of animals are usually reduced under two great oivisions, 

 those of animal life which are concerned with sense and motion, and 

 those of vegetative life which include nutrition and reproduction, 

 and which are common to the vegetable with the animal kingdom. 

 The development of the animal functions may be manifested by a 

 high condition of the organs of sense, and a general perfection of 

 the faculties as far as the character of the type will admit, or by a 

 more special development of the motory powers with the other quali- 

 ties in immediate connection with them, thus forming two distinct 

 plans of development connected with the higher attributes of animal 

 life. In reference to the vegetative life, we may have a higher and 

 a lower grade — the latter being the lowest condition consistent with 

 the type ; and also a case for anomalous modes of obtaining and 

 appropriating nourishment, usually accompanied by elongated forms 

 and peculiar habits. These five distinct plans of development may 

 all be worked out in connection with each different type of structure, 

 and the effect is, that whilst the common type establishes affinity, 

 with variations which are commonly expressed by a circular arrange- 

 ment, a relationship is also perceived between each form and the 

 corresponding mode of development of every other type, producing 

 that complex network of relations which is recognized in nature, and 

 showing how the most marvellous variety is consistent with har- 

 monious order and the prevalence of fixed law. To affirm that there 

 is no other conceivable mode of development of a type of structure 

 than will readily come under one of the five tendencies above 

 enumerated might be rash ; but to affirm that these are really mani- 

 fested, are sufficient for the purpose, and consistently explain the 



