The incomprehensible minuteness and vast numbers of the physiological germs or atoms (which themselves must be compounded of numbers of Spencer's physiological units) is the only difficulty; but that is only on a par with the difficulties in all conceptions of matter, space, motion, force, etc.
As I understood Spencer, his physiological units were identical throughout each species, but slightly different in each different species; but no attempt was made to show how the identical form of the parent or ancestors came to be built up of such units.
LETTER 219.

TO A.R.WALLACE.

Down, February 27th [1868].
You cannot well imagine how much I have been pleased by what you say about pangenesis.

None of my friends will speak out, except to a certain extent Sir H.Holland, who found it very tough reading, but admits that some view "closely akin to it" will have to be admitted.

Hooker, as far as I understand him, which I hardly do at present, seems to think that the hypothesis is little more than saying that organisms have such and such potentialities.