[224]. It is generally known that the earlier physiologists believed in what was called the ‘evolutionary theory,’ or the ‘theory of preformation.’ This assumes that the germ contains, in a minute form, the whole of the fully-developed animal. All the parts of the adult are preformed in the germ, and development only consists in the growth of these parts and their more perfect arrangement. This theory was generally accepted until the middle of the last century, when Kaspar Friedrich Wolff brought forward the theory of ‘epigenesis,’ which since that time has been the dominant one. This assumes that no special parts of the germ are preformations of certain parts of the fully-developed animal, and that these latter arise by a series of changes in the germ, which gradually gives rise to them. In modern times the theory of preformation has been revived in a less crude form, as is shown by the ideas of Nägeli, and by Darwin’s ‘pangenesis.’—A. W., 1888.
[225]. Nägeli, l. c. p. 110.
[226]. See Darwin, ‘The Variation of Animals and Plants under Domestication.’ 1875. Vol. I. p. 311.
[227]. Appendix to page [290].
[228]. Weismann, ‘Naturgeschichte der Daphnoiden,’ Zeitschrift f. wiss. Zool. XXIII. 1879.
[229]. Appendix to page [277].
[230]. Compare W. K. Brooks, ‘The Law of Heredity, a Study of the Cause of Variation, and the Origin of living Organisms.’ Baltimore, 1883.
[231]. l. c., p. 82.
[232]. This seems to be the general opinion (see the quotation from Huxley in Brooks’ ‘Heredity,’ p. 127); but I rather doubt whether there is such a constant difference between mules and hinnies. Furthermore, I cannot accept the opinion that mules always resemble the ass more than the horse. I have seen many mules which bore a much stronger likeness to the latter. I believe that it is at present impossible to decide whether there is a constant difference between mules and hinnies, because the latter are very rarely seen, and because mules are extremely variable. I attempted to decide the question last winter by a careful study of the Italian mules, but I could not come across a single hinny. These hybrids are very rarely produced, because it is believed that they are extremely obstinate and bad-tempered. I afterwards saw two true hinnies at Professor Kühn’s Agricultural Institute at Halle. These hinnies by no means answered to the popular opinion, for they were quite tractable and good-tempered. They looked rather more like horses than asses, although they resembled the latter in size. In this case it was quite certain that one parent was a stallion and the other a female ass.—A. W. 1889.
[233]. Darwin, ‘Variation of Animals and Plants under Domestication,’ 1875, Vol. II. p. 41.