Sir John Lubbock’s Address. Zico 
life. In such cases as these external conditions act upon the 
larvee as they do upon the mature form; hence we have two 
classes of changes, adaptational or adaptive, and developmental. 
ese and many other facts must be taken into consideration ; 
nevertheless naturalists are now generally agreed that embryo- 
logical characters are of high value as guides in classification, 
and it may, I think, be regarded as well-established that, just 
as the contents and sequence of rocks teach us the past history 
of the earth, so is the gradual development of the species indi- 
ora b the structure of the embryo and its developmental 
changes. 
When the supporters of Darwin are told that his theory is 
incredible, they may fairly ask why it is impossible that a 
species in the course of hundreds of thousands of years should 
have passed through changes which occupy only a few days or 
weeks in the life-history of each individual 
The phenomena of yolk-segmentation, first observed by 
Prevost and Dumas, are now known to be in some form or 
other invariably the precursors of embryonic development ; 
while they reproduce, as the first stages in the formation of the 
higher animals, the main and essential features in the life-his- 
tory of the lowest forms. The “ blastoderm” as it is called, 
or first germ of the embryo in the egg, divides itself into two 
layers, corresponding, as Huxley has shown, to the two layers 
into which the body of the Ccelenterata may be divided. Not 
only so, but most embryos at an early stage of development 
have the form of a cup, the walls of which are formed by the 
two layers of the blastoderm. Kowalevsky was the first to 
show the prevalence of this embryonic form, and subsequently 
Lankester and Heckel put forward the hypothesis that it was 
the embryonic repetition of an ancestral type, from which all 
the higher forms are descended. The cavity of the cup is sup- 
posed to be the stomach of this simple organism, and the open- 
ing of the cup the mouth. The inner layer of the wall of the 
cup constitutes the digestive membrane, and the outer the skin. 
To this form Heckel gave the name Gastrea. It is, perhaps, 
doubtful whether the theory of Lankester and Heckel can be 
accepted in precisely the form they propounded it; but it has 
ad an important influence on the progress of embryology. I 
cannot quit the science of embryology without alluding to the 
very admirable work on “Comparative Embryology” by our 
hew general secretary, Mr. Balfour, and also the " Klements of 
Embryology” which he had previously published in conjunc- 
tion with Dr. M. Foster. 
Th 1842, Steenstrup published his celebrated work on the 
“ Alternation of Generations,” in which he showed that many 
Species are represented by two perfectly distinct types or 
