38 Greek Biology 



justice of his comparison to and contrast with the mammalian 

 mode of development. 1 The work of Johannes Miiller at once 

 had the effect of drawing the attention of naturalists to the 

 importance and value of the Aristotelian biological observations. 



Aristotle attempts to explain the viviparous character of the 

 Selachians. His explanation has perhaps little meaning for the 

 modern biologist, just as many of our scientific explanations 

 will seem meaningless to our successors. But such explanations 

 are often worth consideration not only as stages in the historical 

 development of scientific thought, but also as illustrating the 

 fact that while the ultimate object of science is a description of 

 nature, the immediate motive of the best scientific work is 

 usually an explanation of nature. Yet it is usually the descrip- 

 tive, not the explanatory element that bears the test of time. 



' Birds and scaly reptiles ', says Aristotle, ' because of their 

 heat produce a perfect egg, but because of their dryness it is 

 only an egg. The cartilaginous fishes have less heat than these 

 but more moisture, so that they are intermediate, for they are 

 both oviparous and viviparous within themselves, the former 

 because they are cold, the latter because of their moisture ; for 

 moisture is vivifying, whereas dryness is farthest removed from 

 what has life. Since they have neither feathers nor scales such 

 as either reptiles or other fishes have, all of which are signs 

 rather of a dry and earthy nature, the egg they produce is soft ; 

 for the earthy matter does not come to the surface in their eggs 

 any more than in themselves. That is why they lay eggs in 

 themselves, for if the egg were laid externally it would be 

 destroyed, having no protection.' 2 



This explanation is based on Aristotle's fundamental doctrine 

 of the opposite qualities, heat, cold, wetness, and dryness, 

 that are found combined in pairs in the four elements, earth, 

 air, fire, and water. The theory was of the utmost importance 



1 Johannes Miiller, Ueber den glalten Hai des Aristoteles, Berlin, 1842. 



2 De generatione animalium 9 ii. i ; 733 a 6. 



