Bilateralism. 149 



tcuy organs with the bilateral activities, and must have been double 

 from the start. With the lungs, however, the case is different. The 

 ^mbryological development of the vertebrate lung is from a single air 

 sac, which divides into two, each of which becomes a lung. Without 

 doubt this was the order of the tribe development of that organ. The 

 original of that single air sac is illustrated by the single air bladder of 

 the bony fishes, while the original of the double sac stage has an illus- 

 tration in the double air bladders of the ganoid fishes, as the Polypterus. 

 The modification of such air sacs into lungs, and the change ' from 

 breathing the air in the water to breathing the air directly from the at- 

 mosphere are much less violent than one would naturally suppose, as 

 we shall see further on. Among other double organs that may be 

 named, are the thymus and thyroid glands, which have been explained 

 in Chap. V. These are organs concerned in the nutrition of the lower 

 races and the young of higher races, including Man, but which become 

 functionless in the adult Man. The two lobes of the thyroid are joined 

 by a narrowing called the isthmus. The tear glands are paired to cor- 

 respond with the eyes. The salivary glands are in pairs, three or four 

 on each side of the mouth and jaws. The milk glands are in pairs, 

 usually one on each side, but not unfrequently two or three on a side. 



The foregoing examples appear to point to the following generaliza- 

 tion : that, as a rule, organs developed directly from the outer tube of 

 the body are, from the first, in duplicate, while those developed from 

 the inner tube alone are at first single. The subsequent modification of 

 single into double organs is due to the more or less independent action 

 of the two sides each of which establishes relations with a definite half 

 of the single organ, the neutral or inactive tract, between which two 

 halves will first become fixed membranous tissue crystallizing into a 

 septum, and finally a double one, making a permanent division of the two. 



The subsequent modification by which two primary organs tend to 

 become consolidated into one may arise from more than one cause. 

 First, where the bilateral action is entirely cooperative and not antago- 

 nistical, or in cases where the bilateral functions have ceased, as, for 

 example, in the urinary bladder, which, starting as mere dilations of 

 the two ureters, becomes single by the absorption of the walls between 

 the two. 



The function of the bladders, as mere receptacles of the waste, is 

 chiefly a passive function, and economy of room and material, both of 

 which are important instruments of selection, will tend, in the course of 

 time, to the elimination of the useless wall and the establishment of the 

 economical, spherical or ovoid shape. The same considerations apply 

 to the development of the single from the double uterus. 



A second cause of the reduction of duplicate organs from two to one 



