"Many ingenious philosophers have found so great difficulty in conceiving the manner of the reproduction of animals that they have supposed all the numerous progeny to have existed in miniature in the animal originally created, and that these infinitely minute forms are only evolved or distended as the embryon increases in the womb. This idea, besides being unsupported by any analogy we are acquainted with, ascribes a greater tenuity to organized matter than we can readily admit" (p. 317); and in another place he claims that "we cannot but be convinced that the fetus or embryon is formed by apposition of new parts, and not by the distention of a primordial nest of germs included one within another like the cups of a conjurer" (p. 235). 9. To explain instinct he suggests that the young simply imitate the acts or example of their parents. He says that wild birds choose spring as their building time "from the acquired knowledge that the mild temperature of the air is more convenient for hatching their eggs;" and further on, referring to the fact that seed-eating animals generally produce their young in spring, he suggests that it is "part of the traditional knowledge which they learn from the example of their parents."[156] 10. Hybridity. He refers in a cursory way to the changes produced by the mixture of species, as in mules. Of these ten factors or principles, and other views of Dr. Darwin, some are similar to those of Lamarck, while others are directly opposed. There are therefore no good grounds for supposing that Lamarck was indebted to Darwin for his views. Thus Erasmus Darwin supposes that the formation of organs precedes their use. As he says, "The lungs must be previously formed before their exertions to obtain fresh air can exist; the throat or oesophagus must be formed previous to the sensation or appetites of hunger and thirst" (_Zoonomia_, p. 222). Again (_Zoonomia_, i., p. 498), "From hence I conclude that with the acquisition of new parts, new sensations and new desires, as well as new powers, are produced" (p. 226). Lamarck does not carry his doctrine of use-inheritance so far as Erasmus Darwin, who claimed, what some still maintain at the present day, that the offspring reproduces "the effects produced upon the parent by accident or cultivation." The idea that all animals have descended from a similar living filament is expressed in a more modern and scientific way by Lamarck, who derived them from monads. The Erasmus Darwin way of stating that the transformations of animals are in part produced by their own exertions in consequence of their desires and aversions, etc., is stated in a quite different way by Lamarck.