476 The American Naturalist. [June, 
of frogs, Rana temporaria and Rana esculenta, behave very dif- 
ferently toward certain drugs, as Schmiedeberg, Monnier, Vul- 
pian, Harnack and Meyer, and others have shown. In R. tem- 
poraria, caffeine causes a decrease in excitability ; in R. escu- 
lenta an increase; in R. temporaria pilocarpine causes paraly- 
sis; in R. esculenta tetanus. The venom of onesnake is harm- 
less for its own species, but poisonous for others. The spinal 
cord of the fish is differently endowed from that of the frog, 
though the differences have never been properly investigated. 
The muscle of the Insect is far removed functionally from that of 
the Crustacean, though how far remains to be discovered. I do 
* not overlook the fact that already much excellent work upon the 
physiology of the Invertebrates and lower Vertebrates has been 
done, but too often such work has not been comparative. Fit- 
ness for the object of the research is the usual determinant of 
choice—and hence the frog has taught us most of our physt- 
ology of muscle. Sooner or later this must all be changed, the 
functional differences must be made known, and the exact 
position of each plant, each Invertebrate, and each V ertebrate, 
in the physiological series, together with the exact position of 
his organs and tissues and cells must be understood. For we 
must recognize the fact that function in any one species has 
come to be—an evolution of function is as much a reality asan 
evolution of form. The adult body and its organs, tissues and 
cells are the functional derivatives of the germ-cells—in the 
growth of the individual there has been a physiological onto- 
geny. Soin the growth of the species there has been a ant 
gressive or retrogressive development of function ; and one 0 
the most attractive fields for our future work will be the trac- 
ing out of the phylogeny of function, now a practically un- . 
known subject? The difficulty of such an undertaking 1 
great, for the rich palaeontological series is beyond the tea 
of the experimentalist. Yet this should be no bar to the sys- 
tematic investigation of existing forms. Such a phylogeny 
will vary with each functional part (organ, tissue oT cell); @9o 
| y netional part (organ, poretory 
if, in one genus, certain brain functions and certam $ p 
functions are always found, the presence of the same 
* Cf. Dohrn, Das Princip des Functionswechsel. 
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