366 THE STUDY OF ANIMAL LIFE CHAP. 



to some ancestor or animal. So far as this idea persists 

 in the minds of civilised men, it is much purified and 

 sublimed. It raises the difficult question of the relation 

 of mind and body. 



(b) Metaphysical Theories. For a time it was com- 

 mon to appeal to ' vires formativce," " hereditary ten- 

 dencies," and " principles of heredity," by aid of which 

 the germ grew into the likeness of the parent, and this 

 tendency to resort to verbal explanations is hardly to 

 be driven from the scientific mind except by intellectual 

 asceticism. 



(c) Mystical Theories. During the eighteenth cen- 

 tury and even within the limits of the enlightened nine- 

 teenth, a quaint idea of development prevailed, according 

 to which the germ (either the ovum or the sperm) con- 

 tained a miniature organism, preformed in all trans- 

 parency, which only required to be unfolded (or "evolved," 

 as they said), in order to become the future animal. More- 

 over, the egg of a fowl contained not only a micro- 

 organism or miniature model of the chick, but likewise in 

 increasing minuteness similar models of future genera- 

 tions. Microcosm lay within microcosm, germ within 

 germ, like the leaves within a bud awaiting successive 

 unfolding, or like an infinite juggler's-box to the " evolu- 

 tion " of which there was no end. This " preformation 

 theory " or " mystical hypothesis " was virtually but not 

 actually shattered by Wolff's demonstration of " Epi- 

 genesis ' or gradual development from an apparently 

 simple rudiment. But the prcformationists were right in 

 insisting that the future organism lay (potentially) within 

 the germ, and right also in supposing that the germ in- 

 volved not only the organism into which it grew, but its 

 descendants as well. The form of their theory, however, 

 was crude and false. 



(d) Theories of Pangenesis. Scientific theories of here- 

 dity really begin with that of Herbert Spencer, who in 

 1864 suggested that " physiological units " derived from 

 and capable of growth into cells were accumulated 

 from the body into the reproductive elements, there to 

 develop the characters of structures like those whence 



