260 MOSTLY MAMMALS 
exists only in the species inhabiting the plains. Moreover, 
it is only in South Africa that the transitional form is met 
with, and only in the north of the continent that the 
striping has been completely lost. 
But, as I have already mentioned in earlier articles, this 
is only one phase of a general tendency among mammals 
to replace their spots or stripes by a uniformly coloured 
coat. 
So far as I am aware, no one has ever attempted to 
give a philosophical reason for this remarkable tendency. 
But till an adequate explanation of the phenomenon be 
forthcoming, naturalists, to repeat the words of a well- 
known ornithologist, have left half their work (and I am 
inclined to think the more important half) undone. Without 
ascertaining the reason for phenomena of this nature, our 
zoological work is, indeed, as though a man were content 
with describing the mechanism of a complicated machine 
without an inkling as to its use. 
One word more, and I have done. To the systematic 
zoologist, the quagga is an animal of special interest as 
affording evidence of the intimate relationship between 
the zebras and the wild asses. Although, judging from 
its geographical distribution, it was probably not the actual 
transitional form between the two groups, yet it serves to 
show the manner in which the transition was effected. 
