22 THE INTERNATIONAL MONTHLY. 
One of the greatest services which the theory of evolution has 
rendered to the natural sciences is the manner in which it has 
unified them, has given them a common point of view, and has 
showed that they all deal with different aspects of the same great 
problem. In preevolutionary days the various branches of zool- 
ogy were entirely independent of one another, and could have 
had, at best, but an ideal or metaphysical connection. Morphol- 
ogy was one science, descriptive zoology another, geographical 
zoology a third, and palaeontology a fourth. As long as it was 
believed that every species of animal was practically unchange- 
able and had been separately created, no history or explanation 
of a species was possible, nor could any rational account of its 
geographical distribution be given. Why certain regions of the 
world resembled each other in their animal and vegetable produc- 
tions, and differed widely from other regions was an unanswerable 
question, which was not to be explained by a reference to the 
fossils ; for the fossils, like the living organisms, had been them- 
selves immutable, and could have had no material connection with 
modern types. Palaeontology, indeed, was merely the dry and 
lifeless record of successive creations that had only an ideal rela- 
tion with one another. That fossils became more and more like 
existing organisms in proportion as the recent epoch was 
approached, could have only a metaphysical significance, and 
afforded no explanation of the present order of things. In short, 
the Linnaean dogma of the immutability of species acted as an 
impassable barrier to any real understanding of the living world. 
Creation was an ultimate fact, beyond which it was not possible 
to penetrate. 
The general acceptance by naturalists of the theory of evolu- 
tion as a working hypothesis changed all this as if by the stroke 
of a magician's wand, and reduced the apparent chaos of uncor- 
rected parts to an orderly array. Granting that species have 
arisen naturally by descent with modifications from older species, 
it at once becomes evident that all the sciences dealing with life 
are but parts of one great whole, their common task being the 
explanation of the present order of things, the determination of 
