14 



sufficient food, increase indefinitely in size from time to time ; but 

 I am not here dealing with growth but rather with the length of 

 time which is required to develope genetically an independent 

 existence, and the further time occupied before maturity is reached. 



So very little is positively known about the reproduction of 

 Fishes, that my observations on the subject must be brief. 



Take, however, for illustration, the small fish with which we 

 are all familiar, the Stickleback, Gasterosteus aculeatus. 



In this species the young of one year are adult, and themselves 

 parents in the next. 



Compare this fish with the Salmon and it will at once be seen 

 how much longer time is required to develope an adult Salmon. 



Willoughby says that the Salmon requires six years to attain its 

 fall growth, and that the yeai-ly stages were so well marked that 

 at each season the fish received a different name among the fisher- 

 men on the river Eibble ; these names he gives as Smolt, Sprod, 

 Mort, Fork-tail, Half Fish, and Salmon ; but there is reason to 

 believe that well fed fish become mature in half the time stated 

 by Willoughby. 



Still the rule holds good that the little Stickleback becomes 

 mature in a much shorter time than the Salmon. 



The Sharks, the highest developed fishes, resemble in many re- 

 spects the mammalia ; several of the genera are viviparous, and 

 appear also to take care of their young when in an immature 

 condition. 



Many of the Sharks grow to an enormous size, often weighing 

 several tons, and fi'om the great difference in size of those captnred 

 I apprehend maturity is slowly arrived at. 



Passing over the Reptiles and Batrachians, the Birds form the 

 next class for consideration. 



Here the postulate, that the larger the size the longer the time 

 required for development, can, I think, be logically proved. 



Amongst Birds, however, two mdely different conditions of 

 development obtain, although all are hatched fi-om eggs yet the 

 young birds so produced differ essentially. 



Hesthogenous Birds produce their young clothed, with their 

 eyes open ; and from the first day of their existence capable of 

 feeding themselves. 



Gymnogenous Birds on the other hand produce their young 

 naked, blind, helpless, and quite incapable of feeding themselves. 



The period of time occupied in incubation, in both divisions of 



