b 
For the sake of illustration, we assume that a particular Fauna 
or Flora, or a particular group of individualities, or, in the case of 
the Comparative Anatomist, a particular system of structures (such 
as the organs of circulation) engages for a time his exclusive attention. 
In any of these attitudes the Biologist is precisely in the posi- 
tion of a traveller mapping out the features of an unknown con- 
tinent. By careful observation, powerfully developed by artificial 
methods of survey, he makes himself acquainted with all the phe- 
nomena presented within the limited area of his wanderings, and, 
*>y a comparison based chiefly on analogy, deduces a series of 
general propositions or conclusions in regard to that which lies 
beyond his reach. Experience has shown that in this way a 
correct and comprehensive generalization may be established re- 
specting the actual condition of the continent in its entirety, 
although, of course, the traveller’s conceptions of the minor features 
presented by unexplored districts must necessarily remain imper- 
fect, or altogether fallacious. How is this ? Simply, I argue, 
because, although the laws regulating the phenomena of nature 
over all parts of the continent are constant and invariable, the 
degree , direction , and conditional circumstances under which they 
operate may, and often do, produce such strangely-modified 
results, that when we descend to particulars we are sure to meet 
with the most irregular, varied, and unlooked-for phenomena. 
Let us now apply this train of reasoning in the case of the 
Comparative Anatomist, selecting for special illustration Dr J. C. 
Yan Hasselt’s discovery of the peculiar mode of blood-circulation 
which obtains in Salpa (fig. 1) — a genus of tunicated mollusks. 
Fig. 1. 
The form of Salpa known as S. maxima , as represented by Milne-Edwards. 
According to the philosophical explanations of Prof. Huxley,, this outline 
should be reversed, in which case the parts are as follows :—a } inferior lip of 
the anterior respiratory aperture ; &, posterior breathing orifice ; c, abdomen 
containing the visceral nucleus ; d, branchial lamina or gill ; e , heart and 
dorsal vessel ; /, nerve-ganglion, or so-called oculiform point; g , g, appendages 
of the test. 
The direction of the circulating current alternates, the flux and reflux 
occurring at regular intervals. 
The student of the circulation, who during years of investigation 
found this life-stream flowing through the ordinary channels in one 
invariable direction, has suddenly become acquainted with an 
B 2 
