Bilateralism. 147 



part of the brain, is double, right and left, in all the higher vertebrates, 

 while it is undivided in the lowest. The lungs start from the air blad- 

 der of fishes, which is at first a single sac, but later is found double. 

 In the Ceratodus and Lepidosiren these bladders become true double 

 lungs. In the Cyclostomi the nose is a single tube, but in the early 

 fishes ( Selachians ) this is divided into two, remaining such in the higher 

 animals. In the Cyclostomi the sexual glands, ovaries and testes, are 

 paired, but no supplemental organs are required as the impregnation 

 takes place after the eggs are laid or thrown off. 



The same is true of the Amphibians. But in the Monotremes, Or- 

 nithorhynchus, &c. , with which the impregnation is internal, additional 

 organs became essential, and these were developed in pairs. Each 

 ovary of the female was supplemented by a uterus, and each testis of 

 the male by a penis. These double organs persist, more or less com- 

 pletely, in the most of the genera of the Marsupials, as the Opossum, 

 Wombat, Kangaroo, &c. From the Kangaroos up, these double organs 

 have become partly united to form single ones. In the Lemurs the 

 two uteri are united at their lower ends, leaving the upper ends still 

 free. In the Apes and Man they are consolidated into one, and the only 

 suggestion of their duplex nature remaining to these races, is in the two 

 fallopian tubes entering the upper opposite corners ; the representatives 

 of the upper ends of the ancient Miillerian ducts ; the united uteri 

 with their outlet representing the lower ends of those ducts. But the 

 organs all still retain marks of their duplex origin. 



In some of the ganoid fishes as the Amia calva the urinary bladder 

 is a double organ, one bladder being developed in connection with each 

 kidney by the dilation of the ureter. In the Selachian fishes the ureters, 

 or ducts leading from the kidneys, are dilated, but not to the extent of 

 bladders, while in the Cyclostomi Lampreys and Myxine there are 

 simply the two ureters opening behind the rectum. On the other hand, 

 in the bony fishes ( Teleosteans } the two urinary bladders of the ganoids 

 are consolidated into one bladder which receives the two ureters, and 

 which still opens behind the rectum. In all the higher vertebrata the 

 single urinary bladder prevails, but the natural original position of the 

 external openings is reversed with reference to each other, the urogen- 

 ital opening being in front instead of behind the anal. In this case, 

 then, it would seem that the organs beginning as duplex have tended to 

 a consolidation into one. And the same may be said of the reproduc- 

 tive organs, some of which have thus become consolidated from two in- 

 to one, while others still remain in duplicate. Some further examples 

 may be mentioned. In many genera of the Rodents, as the Rabbits, 

 for example, the uterus of the female is still completely two, each of 

 which opens separately into the vagina. The male Rabbits have two 



