Devonian Fishes of Campbelltown and Scaumenac Bay, 125 
jaw behind. Along the anterior two-thirds of the outer 
margin is articulated, a row of three ossicles bearing the 
upper laniary teeth, and thus corresponding to the mandibular 
internal dentary ossicles on which I long ago showed that 
the laniaries of the lower jaw (except the anterior one) were 
borne in Rhizodus. There are two vomers placed behind 
the premaxillee, each of which, as in the Osteolepide, bears a 
large laniary tusk. -As in the Holoptychiide, Rhizodontide, 
and Osteolepidee in general, the maxilla proper bears only 
very small teeth, 
XIII. Heredity and its Bearings on the Phenomena of 
Atavism. By Gusrav Many, Esq., M.B., C.M., Physio- 
logical Laboratory, University, Kdinburgh. 
(Read 15th February 1893.) 
Gebt mir Materie und ich will eine Welt daraus bauen.—Kanv. 
How is it that we resemble, and yet differ from our 
parents? What factors are at work during the union of 
the sperm with the ovum, to which union Fol has applied 
the term “fecundation”? Why should a single cell, a spore, 
derived asexually from a metazoon or a metaphyton, be 
capable of development into an organism with specific 
characters? According to what laws can a new individual 
arise by budding or by simple fission, and how can a part 
of an individual, if separated artificially, reproduce the whole 
organism ? 
A consideration of these questions leads up to two 
others: (1) given a series of species which reproduce 
themselves by fission, spore-formation, and fecundation, 
repectively, are we justified in assuming that the variability 
of the offspring is greatest in those individuals which owe 
their existence to a sexual process? and (2) how does this 
variability lead to the recurrence of ancestral traits, to which 
latter phenomenon the name of Atavism has been given ? 
These various questions I shall endeavour to answer from 
1 Also read before the Darwinian Society, Edinburgh, on 9th November 1892. 
