THE EMBRYOLOGICAL CRITERION 127 



develop from the same " fundamental organ," and in the last 

 resort from the same germ-layer, have a certain kinship, 

 which may even reach the degree of exact homology. 



Now since the mode of development in each type is 

 peculiar to that type, organs of the same name in different 

 types must not necessarily be accounted homologous, even 

 if they correspond exactly with one another in their 

 general functional relations to the rest of the organs. 

 Thus the central nervous system of Arthropods must not be 

 homologised with the central nervous system of Vertebrates, 

 for it develops in a different manner. So, too, the brain of 

 Arthropods or of Mollusca is not strictly comparable with 

 the brain of Vertebrates. Again, the air-tubes or tracheae 

 of insects are, like the trachea and bronchi of many 

 Vertebrates, air-breathing organs. But the two organs are 

 not homologous, for the air-tubes of Vertebrates are developed 

 from the alimentary tube (" fundamental organ " of the 

 alimentary system, developed from the vegetative layer), 

 while the air-tubes of insects arise either by histological 

 differentiation, or by invagination of the skin (p. 236). Organs 

 can be homologous only within the limits of the big groups; 

 there can be no question of homology between members 

 of different types. 



The development of plants, like the development of 

 animals, is essentially a progress from the general to the 

 special (p. 242). Botanists have not been troubled by any 

 recapitulation theory, and in founding their big groups, 

 Acotyledons, Monocotyledons, and Dicotyledons, upon 

 embryological characters, they were guided by true 

 principles, which ought indeed to be followed in zoology. 

 If we knew the development of all kinds of animals 

 sufficiently well, then the best way to classify them would 

 be according to the characters they show in their early 

 development, for it is in early development that they show 

 the characters of the type in their most generalised form. 

 As it is, we have in our ignorance to establish the big 

 groups by the study of adult structure, but we find, on 

 putting together all we know of comparative embryology, 

 that a classification of animals according to the mode of 

 their development gives, as is only natural, the same four 



