Lazvs oj Incubation and Gestation. 270 



as conipaved with another. This causes the value of n to be 

 high in ruminants, and higher still in Proboscidea ; while in 

 Carnivora and llodentia it is low, but of approximately equal 

 value, 41 for the first; 35 for the second. 



The first law, stated in its most genei-al form, is this : — " For 

 animals of tlie same size the time of embryo development is 

 inversely proportional to the square of the temperature, that 

 temperature being reckoned from a definite point." 



The second law, similarly stated, is that: — "At the same 

 temperature, the period of development is directly proportional 

 to the sixth root of the weight of the mature animal." 



This latter law is capable of a certain simplification. If two 

 animals are of different sizes, but of the same shape, the weights 

 of their bodies are proportional to the cubes of their lengths. 

 The law in that case would be : — At the same temperature, 

 among animals of the same shape, the period of development is 

 directly proportional to the square root of the length. 



Thus we have 



t = n^ 7 

 but this is the same as 



where f= - 



Now this is the well-known equation for the space tra\ersed 

 Ijy a body moving under the influence of a constantly accelerating 

 force, and the significance of the law therefore is tliat if we consider 

 the germinal point as the starting place, and imagine the embryo 

 to travel outwards from it to the periphery, the velocity of the 

 motion will be such as would result from a constantly accelerating 

 force propelling it from the germinal spot outwards. 



In the appendix a list is given of the gestation periods of the 

 rodents, the family Leporidse being set down apart from the 

 others as requiring a lower value of the constant. The Beaver is 

 a very aberrant case. The only four species of Perissodactyls for 

 which I can get information form a fairly consistent group. For 

 them the value of the constant is very high, but in the Probos- 

 cidea it rises higher than in any other of the lower families, 

 reaching a value of 120. In the Prosimia it appears to be only 

 equal to the value of n in the ruminants; but in the Quadrumana 



