APPENDIX. 
13-s 
any extent compatible with life. This is explained by the 
first variation affecting a few only of the ids of the germ plasm. 
Let us suppose for the sake of convenience that there were 
twenty ids in the germ plasm, and that, say, three of them were 
so modified as to produce cresting. Then, at the next generation 
we get a reducing division. It might happen that the crested ids, 
as 1 may call them, might be thrown out in the polar bodies, in 
which case we should get a return to the normal form. On the 
other hand, it might happen that ten normal ids of the egg cell 
might be thrown out, leaving seven normal and three crested. If 
this egg cell should happen to be fertilised by a sperm cell which 
had similarly thrown out all normal ids, we should get a new 
germ plasm, containing fourteen normal ids and six crested ones, 
in which the crested character would be intensified in the next 
generation. 1 may give you an ocular illustration of this by 
supposing the ids to be represented by coloured discs. I show 
you here the result of several different reducing divisions and 
recombinations of ids. ( Blackboard demonstration .) 
Let us now glance for a moment at the phenomenon of 
reversion or atavism. You all know that an organism will some- 
times exhibit characters and peculiarities which cannot be detected 
in either of its parents, but which belonged to one or other of 
its grand-parents, or even to some ancestor many generations 
back. The explanation of this in Weismann’s theory is very 
simple. I have already explained that the germ plasm of an 
individual contains ids, derived from a long line of ancestors. 
Every variety of recent origin probably contains some normal 
ids of the species to which it belongs. Our friend Mr. Druery 
can tell us that in raising young plants of A. f. f. Clarissima some 
of the offspring tend to run back to the normal form of A. filix- 
fcemina. The reason for this is that by a reducing division the 
germ cells have got rid of some of the Clarissima ids, and so the 
normal ids have obtained a majority and gained the upper hand. 
There are probably, however, some Clarissima ids still contained 
in the germ plasm of these degenerate forms, but these form 
only a small minority and are consequently unable to produce 
much impression upon the character of the plant. If, however, 
we raise again from these renegades we get a fresh series of 
reducing divisions, and in some of these it may happen that 
the Clarissima ids gain the upper hand, and so we again get a 
few Clarissima seedlings from the degenerated parents, although 
the bulk of the offspring are probably normal Athyrium filix- 
foemina. 
Let us take as another example Polypcdium v. Cornubiense, 
which produces normal fronds among ihe dissected ones. In this 
Fern the abnormal ids have presumably only a very small working 
