WILLIAM G. STEVENSON. 13 
knowledge is as yet too imperfect and fragmentary to 
admit of a classification of biological! phenomena which 
will represent a complete history of adult life as related 
to its personal and ancestral development. So long as 
the dogma of the ‘‘ constancy of species’’ was accepted 
—which ‘excludes any natural relationship”? between 
different species—it was possible to make a morphologi- 
eal classification which would satisfy the intellectual 
demands of the days of either Aristotle or Linnzeus. 
The sciences of comparative anatomy and embryology 
were then unknown, and such important factors, in the 
analysis or synthesis of organic life, as ‘ differentiated,” 
‘homologous’? and ‘‘rudimentary”’ organs had no 
meaning; neither was there knowledge of the trans- 
forming influence of environment nor of the potency of 
inheritance, in fixing and transmitting the changes 
wrought in the organism. 
Cuvier knew not the methods of development, but 
only the anatomy of the matured structure. His inves- 
tigations, however, proved the existence of extinct forms 
of life, whose characters were intermediate between dis- 
tinct groups of existing forms, and therein were laid 
the foundations of paleeontology. 
Bichat inaugurated the study of tissues, and the 
functions or dynamics of the bodily organs. 
Von Baer, in 1827, discovered the mammalian ovule, 
made embryology the basis of anatomical classification 
and established the biological ‘‘law of differentiation 
froma general towards a special form; while Agassiz 
showed that ‘‘in some cases, the older forms preserve, 
as permanent features, structural characters which are 
embryonic and transitory in their living congeners.” 
Then came the patient toil of Darwin in gathering, 
analyzing and classifying the facts which nature so lav- 
ishly furnished to him, until he was able to enunciate 
the theory of variations in nature through *‘ natural se- 
