56 COMPARATIVE ANATOMY CHAP. 



It deals naturally, not only with the bodies of animals now living, but 

 also with the bodies of animals of past geological periods in so far as 

 these are attainable in a fossilised condition ; not only with the bodies 

 of adult animals, but with all the consecutive stages of development of 

 such animals. For an animal form is characterised not only by its 

 structure in a fully grown and sexually mature stage, but by its struc- 

 ture in all the previous consecutive stages of its development. Com- 

 parative anatomy only considers organs, as we have already said, 

 according to their structure and their connections of affinity, not ac- 

 cording to their physiological activities. The relationship of two 

 organs rests upon their descent from the same organ of a common 

 racial form. The proof of this relationship establishes the Homology 

 of the organs in different animals. Thus the anterior or posterior 

 extremities of the Amphibia, Eeptiles, Birds, and Mammals are homo- 

 logous to each other and to the pectoral or ventral fins of the Fish, 

 because these organs have a common origin. The limbs of the Ferte- 

 brata and those of the Arthropoda are not homologous, but only ana- 

 logous, because they cannot be referred to a definite organ in a 

 common racial form. They were first formed independently within 

 each of these groups, and the superficial similarity which they exhibit 

 is only the result of their adaptation to the" same function. 



Zoological research has further proved that in the process of time 

 organs can undertake functions quite different from those which they 

 originally performed (principle of the change of functions). The air- 

 bladder of fishes, for example, is principally a hydrostatic apparatus 

 used by these animals for rising or sinking in the water. At the same 

 time, in certain fishes the air-bladder may also undertake the secondary 

 function of a respiratory apparatus. This secondary function becomes 

 in the higher Vertebrata the chief function ; the lung rises out of the 

 air bladder, and the original function is quite lost. The so-called 

 rudimentary organs are of great importance in comparative anatomy ; 

 these are degenerated organs which are not in the condition to perform 

 any useful function for the organism. They are remains of originally 

 well-developed and functionally important organs, retained by inherit- 

 ance, but in the act of disappearing. Thus the human processus vermi- 

 formis is a small remainder of an intestinal caecum which is greatly 

 developed in certain Mammalia of a lower order and energetically 

 takes part in the work of digestion. 



How is the rise of the lowest, simplest Metazoa to be imagined ? 

 This question is answered by various theories ; one of these, the 

 Gastrsea theory, has been very generally accepted. This theory rests 

 upon two series of facts : 



1. In the development of very many Metazoa there arises, by 

 repeated division of the egg cell, a hollow group of similar cells, 

 which in its structure shows a general correspondence with a 

 Protozoan colony (Polvox, Magosphcera). The cells of this group 



