52 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



indispensable service to the understanding of the complicated corre- 

 lations and the harmonious adjustments in the organic world. 



The same plan of structure and arrangement of the organs is not 

 found, as Geoffroy St. Hilaire asserted in his theory of analogies, 

 in the whole animal kingdom ; but, on the contrary, there are, as 

 Cuvier stated, several plans of organization or types. The term 

 "Type" was applied by Cuvier to the chief, i.e., the most compre- 

 hensive and general divisions of his system ; and each type was 

 distinguished by the sum of the characters of its form and structure. 

 In the essential characteristics of their structure, the higher and 

 lower members of the same type agree, while in the unimportant 

 details they present the most marked differences. The different 

 types themselves do not represent absolutely isolated groups, nor 

 groups which are exactly equivalent to one another, but in a greater 

 or less degree they are related to one another ; this is evident after 

 an examination of the lower forms and a careful comparison of the 

 developmental histories. 



To morphology belongs the task of pointing out the identity of plan 

 under the most diverse conditions of organization and habits of life, 

 not only among animals of the same group but also between those of 

 different groups. This science has for its object the determination 

 of homologies, as opposed to analogies which concern the similarity 

 of function, i.e., the physiological equivalence of organs found in 

 different groups, e.g., the wing of a bird and that of a butterfly. That 

 is to say, it has to trace back to the same primitive structure parts 

 of organisms belonging to the same or different groups, which with 

 a different structure and under deviating conditions of life discharge 

 different functions ; as, for example, the wing of a bird and the 

 fore-limb of a mammal ; and so to show their morphological 

 equivalence. In the same way the organs of similar structure which 

 are repeated in the body of the same animal, e.g., the fore and hind 

 limbs, are designated as homologous. 



THE STRUCTURE AND FUNCTION OF THE COMPOUND ORGANS. 



The vegetative organs comprise the organs of nourishment which 

 are necessary for all living organisms, whether animal or vegetable. 



In the former, however, they gradually and in the most intimate 

 connection with the progressive development of the animal functions, 

 attain a higher and more complicated structure. In animals, the 

 reception of food is followed by its digestion. The substances to be 

 assimilated, which have been made soluble by digestion, enter a 



