The remarkable anatomical relationship of the embryo of _Galeus_ (_Mustelus_) _laevis_ to its mother’s womb was little noticed by naturalists until the whole matter was taken up by Johannes Müller about 1840.[41] That great observer demonstrated the complete accuracy of Aristotle’s description and the justice of his comparison to and contrast with the mammalian mode of development.[42] The work of Johannes Müller at once had the effect of drawing the attention of naturalists to the importance and value of the Aristotelian biological observations. [41] The history of this discovery is given by Charles Singer, _Studies in the History and Method of Science_, vol. ii, Oxford, 1921, pp. 32 ff. [42] Johannes Müller, _Ueber den glatten Hai des Aristoteles_, Berlin, 1842. 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 descriptive, 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.’[43] [43] _De generatione animalium_, ii. 1; 733ᵅ 6.