HEREDITARY INFLUENCE. 
539 
therhood, namely, in twins, we see the same diversity ; and 
this diversity is exhibited in those rare cases where the twins 
have only one body between them. The celebrated twins Rita 
and Christina* were so fused together, that they had only two 
legs between them : two legs and four arms and two heads ; 
yet they were quite different in disposition. The same dis- 
ference was manifested in the celebrated Presburg twins, and 
in the African twins recently exhibited in London. 
It is clear, then, that offspring do not always closely re- 
semble parents ; and it is further clear, from the diversities in 
families, that they do not resemble them in equal degrees. 
Two brothers may be very unlike each other, and yet both 
like their parents ; but the resemblance to the parents must, 
in this case be variable. So that when we lay down the rule 
of constancy in transmission , we must put a rider on it, to the 
effect that this constancy is not absolute, but is accompanied 
by a law of variation. It is the intervention of this law 
which makes hereditary influence a problem ; without it, 
heritage vrould be as absolute as the union of acids with 
bases. 
Some philosophers have tried to explain the law of con- 
stancy in transmission, and its independence of the law of 
variations, by maintaining that it is the Species only, not the 
Individual, which is reproduced. Thus a sheep is always 
and everywhere a sheep, a man a man, reproducing the 
specific type , but not necessarily reproducing any individual 
peculiarities. All sheep resemble each other, and all men 
resemble each other, because they all belong to specific types. 
What does the reader say to this hypothesis? Burdach, 
who adopts it,+ adduces his facts : for example, a dog from 
whom the spleen was extirpated reproduced dogs with perfect 
spleens; an otter, deprived of its fore paws, produced six 
young with their legs quite perfect ; in a word, “ l’idee de 
l’espece se reproduit dans le fruit et lui donne des organes 
qui manquaient au pere ou a la mere.” The hypothesis has 
seemed convincing to the majority of thinkers, but it labours 
under one fatal objection — namely, species cannot reproduce 
itself, for species does not exist. It is an entity, an abstract 
idea, not a concrete fact. It is a fiction of the understanding, 
not an object existing in nature. The thing species no more 
exists than the thing goodness or the thing whiteness. 
Nature only knows individuals. A collection of individuals 
so closely resembling each other as all sheep resemble each 
* See Geoffroy St. Hilaire, * Philosopliie Auatomique,’ vol. ii ; and 
Serres, * Reclierehes de l’Anatomie Traiiscendante/ 
t ‘ Physiologie,’ ii, 245. 
