AUGUST 5, 1915] 
NATURE 

culties as far as possible. Proofs of statements 
are printed in smaller type, and the general argu- 
ment can be followed quite readily. _Misconcep- 
tions of the position of the exponent of the 
Principle are frequent, and the exponent is not 
always free from blame. In these circumstances 
we welcome the explicit statement on p. 117—and 
the statement cannot be criticised—that while the 
Principle of Relativity cannot say what are the 
equations. for bodies the motion of which is not 
uniform, it can say what they may be, and it 
therefore gives a powerful means of devising such 
equations to be tested.by experiment. A similar 
definiteness is found in-connection with the vexed 
problem of rotation. 
The third part of the work is devoted to the 
necessary modifications of mechanical theory, on 
the supposition that the Principle is universal. 
Fundamental points only are dealt with, to the 
welcome exclusion of a mass of material the in- 
terest of which is almost entirely academic. Very 
speculative developments, such as that of Einstein 
in connection with gravitation, are omitted, and 
the author is thereby enabled to give a very clear 
view of the real implications of the Principle. 

THE DEVELOPMENT OF MAN. 
Heredity and Environment in the Development 
of Men. By Prof. E. G. Conklin. Pp. xiv+ 
(Princeton: University Press; London: 
Price 8s. 6d. 
533: 
Oxford University Press, 1915.) 
net. 
DDRESSING a general audience rather than 
a scientific gallery, Prof. Conklin deals in 
six lectures with the development of Man. In 
the first chapter he shows that development is 
the result of intrinsic factors, implied in the 
organisation of the germ-cells, and of extrinsic 
factors, the environmental conditions. The germ- 
cell is a living individuality, and development is 
one of its functions. New materials and qualities 
appear in increasing complexity, and this is due 
to active combinations and interactions under 
environmental influence. The hereditary ger- 
minal organisation is very complex, but develop- 
ment is no mere unfolding or unpacking. It in- 
cludes processes of “creative synthesis.” 
The second chapter is devoted to the germ- 
cells and their organisation. ‘‘ Development is 
progressive and co-ordinated differentiation of the 
germinal organisation, by which it is transformed 
into the adult organisation.” Different kinds of 
substances are formed epigenetically; these are 
localised and isolated by intra-cellular movements 
NO. 2388, VOL. 95] 

and differential cytoplasmic divisions; and they 
are eventually transformed into various  per- 
manent structures. 
The third chapter deals with the modes of in- 
heritance, with special reference to Man. ‘“ Blend- 
ing ” is still retained on trial, but it is shown that 
“the principles of Mendel have served as an 
Ariadne thread to guide science through the maze 
of apparent contradictions and exceptions in which 
it was formerly lost.’’ In the next chapter the 
author treats of the influence of the environment, 
and is more than usually careful in his analysis. 
He distinguishes the different kinds of develop- 
mental stimuli, the different times of their opera- 
tion, and their varied results. There is a useful 
recognition of functional activity as a factor in 
development. ‘“Nurture,”’ it is argued, means 
much as a condition of individual development, but 
little as a transforming factor in evolution. For 
Man, however, with his social heritage, the evolu- 
tion of the environment must corroborate eugenic 
improvements of the breed. In this and in the 
next chapter, which is devoted to a consideration 
of eugenics, the author turns repeatedly to the idea 
that environmental changes, saturating into the 
germ-cells, may incite heritable variations in the 
germinal organisation. The discussion of eugenics 
is restrained and wise. It is possible to improve 
the human breed (1) by preventing the seriously 
defective from reproducing; (2) by cultivating 
pride of race, and discouraging voluntary infer- 
tility on the part of those who have a goodly 
heritage; (3) by increasing opportunities for early 
and favourable marriages; and (4) by carefully 
conserving the best human mutations or heritable 
variations. But along with these eugenic 
endeavours there must go improvements of 
environment and training—including,  pre- 
eminently, the development of control and good- 
will. 
In the concluding pages Prof. Conklin explains 
how he, as a biologist, finds between the fatalistic 
and voluntaristic extremes a middle way which 
leads to’ action. We venture to commend the 
wise and stimulating discussion of the deter- 
minism involved in the creature’s hereditary and 
nurtural relations and of the other side to this. 
The book is marked by sound judgment, firmness 
and clearness of treatment, and a vigorous 
buoyancy. It is to be strongly recommended to 
those who wish to understand the facts of human 
development in their relation to the larger pro- 
blems of human life. The volume is well got up, 
effectively illustrated, and provided with a care- 
fully selected bibliography and an unusually good 
glossary as well as a full index. 
