Gatty Marine Laboratory, St. Andrews, 159 
aeration, plays an important part in the economy of the 
annelid. 
In such a type as Filograna it is not the hard-and-fast 
rule that “like begets like,” but the inherent tendency to 
vary in every particular in the parts indicated is the main 
factor. It is questionable if, with every care, the production 
.of those with opercula or those with the greatly enlarged 
tips to the branchial filaments could always be relied on by 
breeding from suitable parents of each type, unless the 
exact surroundings are obtainable, since the tendency to 
variability is so intense. It is said that varieties replace the 
original species because they are “‘ more perfectly developed 
and more highly organised, and in all respects better 
adapted to secure its safety, and to prolong its individual 
existence and that of the race. Such a variety could not 
return to the original form; for that form is an inferior 
one, and could never compete with it for existence. Granted, 
therefore, a ‘tendency ’ to produce the original type of the 
species, still the variety must ever remain preponderant in 
numbers, and under adverse physical conditions again alone 
survive” *, 
Whilst many examples of the foregoing statement may be 
found in the higher vertebrates, the case of Vilograna, for 
instance, does not seem to fall into line, for here are variations 
so numerous in structure and development and so inter- 
mingled with each other that it is difficult to say which is 
the original form and which the variation, since all forms 
may be found under like conditions. In this connection it 
may be asked what variety of Filograna has a tendency to 
maintain its existence longer than the original species or 
longer than any other variety? Can it be said that those 
with opercula are better fitted to survive than those devoid 
of them, or that those with the distal ends of the branchial 
filaments enlarged into sausage-like masses supplant those 
without them? Are those with eyes and nine pairs of anterior 
setigerous processes enabled to continue the species more 
effectively than those which have no eyes and only five or six 
setigerous processes? There is no proof that any of these 
is in a better position than another—yet Nature does nothing 
in vain; the facility with which variations occur and the 
vast distribution of the species would lead to the belief that 
a clue may yet be found to unravel the mystery. The 
species certainly fluctuates to and froin regard to the organs 
mentioned, but does not progress along any of the lines 
* This and other quotations are taken from the important papers of 
Mr, Darwin and Mr, Wallace in the Proc, Linn. Soc. vol, iii. no. 9, 1858, 
