294 
Proceedings of Royal Soeiety of Edinburgh. [april 2, 
pressures, and these pressures are to he derived from an unequal 
increase of dimensions in different directions. The lav,rs of this 
increase must determine the fundamental occurrences which bring 
about the formation of the body of the higher animals. 
I cannot as yet find any fault in the short chain of these argu- 
ments, and I daresay that they are in full harmony with all our 
notions of other natural processes. Geology also has much to do 
with layer-foldings and with their consequences. The results of 
geological observation accord in many points with those of embryo- 
logy ; the dislocations and the ruptures of layers follow to a con- 
siderable extent the same law in the formation of the earth’s crust 
as in that of our own body. 
My attempts to introduce some elementary mechanical or physio- 
logical conceptions into embryology have not generally been agreed 
to by morphologists. To one it seemed ridiculous to speak of the 
elasticity of the germinal layers ; another thought that, by such 
considerations, we “put the cart before the horse”; and one more 
recent author states, that we have better things to do in embryology 
than to discuss tensions of germinal layers and similar questions, 
since all explanations must of necessity he of a phylogenetic nature. 
This opposition to the application of the fundamental principles 
of science to emhryological questions would scarcely be intelligible 
had it not a dogmatic background. No other explanation of living 
forms is allowed than heredity, and any which is founded on another 
basis must be rejected. The present fashion requires that even the 
smallest and most indifferent inquiry must he dressed in a phylo- 
genetic costume, and whilst in former centuries authors professed to 
read in every natural detail some intention of the creator mundi^ 
modern scientists have the aspiration to pick out from every occa- 
sional observation a fragment of the ancestral history of the living 
wmrld. The task of reading the chapters of this history seems to he 
as easy as collecting specimens of plants and animals, or of making 
microscopical preparations. The last principles of creeds and of 
theories are introduced into every empirical inquiry, and the danger 
is overlooked that even the best established theories frequently put 
a veil on the eyes of the observer, and interfere with the impartiality 
of his observations. 
I should be the last to discard the law of organic heredity, or to 
