Analyses of Books. 
[September, 
588 
whose motives are open to question. If a biologist enters upon 
investigations not with a view to the discovery of the simple 
truth, but in the hope of meeting with facts irreconcilable with 
organic evolution, we are justified in regarding his results with 
suspicion. Nor is it certain that a contrary bias, in favour of the 
doctrine of Descent, may not in other cases have been present. 
Hence, as Prof. Balfour admits, it may be considered doubtful 
whether the time has yet come for all these observations to har- 
monise and arrange in a coherent whole. Still we think our 
readers will agree with us that he merits the thanks of biologists 
for the manner in which he has executed this difficult task. 
The literature of the subject, scattered through the scientific 
journals and the Transactions of learned societies, is of prodi- 
gious extent, as an examination of the bibliographical appendix 
will show. It conveys, at the same time, the unpleasant reflec- 
tion how small a portion of the researches in this field has been 
executed by Englishmen. 
The present work is devoted solely to the embryology of those 
animals now characterised as “ metazoa,” and the subject is 
treated from a morphological rather than from a physiological 
point of view. The objects of Embryology are shown to be 
twofold — to form a basis for phylogeny, the history of the deve- 
lopment of the group, and for organogeny, the doctrine of the 
origin and evolution of organs. Among the subjects specially 
discussed in the former department are the questions how far 
embryology brings to light ancestral forms common to all meta- 
zoa ? how far some special embryonic larval form is constantly 
reproduced in the ontogeny of the members of one or of more 
groups of the animal kingdom, and how far such larval forms 
may be understood as the ancestral type of such groups ? how 
far such forms agree with adult forms, living or fossil ? how far 
organs appear in the embryo or larva which in the adult state 
are either atrophied or lose their functions, but which persist 
permanently in members of other groups or in lower members 
of the same group ? and how far organs in the course of their 
development pass through a condition permanent in some lower 
form ? The author justly remarks that the solution of these 
problems would be greatly simplified if each organism contained 
in itself the full record of its origin. But the law which we 
have quoted above admits of what to the unscientific observer 
seem exceptions. It is, in other words, merely a statement of 
what would occur were it the only influence under which organic 
development takes place. In actual life there are a number of 
interfering conditions. “ The embryological record is almost 
always more or less abridged in accordance with the tendency of 
Nature (explicable on the principle of the survival of the fittest) 
to attain given ends by the easiest means.” Secondary structural 
features are introduced to adapt the larva or embryo for special 
conditions of its existence, and the problems involved are thus 
rendered exceedingly complicated. 
