260 MONTGOMERY. [VoL. XII. 
must demonstrate satisfactorily that it is undergoing a process 
of development. 
The question to be solved is then, first of all: What are our 
criteria of continuing development? Progressive and regres- 
sive development having been sufficiently characterized, it 
remains necessary to produce criteria, whereby we can deter- 
mine whether a given organ (or organism) is at a particular 
time dominated by a process of development, or whether the 
organ (or organism) is not being influenced by a particular 
developing agency, either progressive or regressive. We may 
now consider briefly three reliable criteria of continuing develop- 
ment, namely, (1) domestication, (2) the presence of geograph- 
ical races (subspecies), and (3) migration; no doubt other 
criteria may be found, but these three are sufficient for our 
present purposes. 
(1) Domestication may be taken as a criterion of continuing 
development, since all organisms in a state of domestication 
are being more or less continuously selected by man, with a 
view to their adaptability for certain uses. The development 
induced by human agency is also very energetic, since man’s 
uses for domesticated animals and plants are manifold, and 
since he frequently introduces changes in their environment. 
And further, as we know in many cases that the length of 
time necessary for the production of a new “ breed”’ has been 
comparatively short, we must conclude that not only was the 
action of the development continuous, but also that it must 
needs have been very energetic. 
(2) The presence of geographical races may also be considered 
a criterion of continuing development. A species is said to pre- 
sent geographical races or subspecies when in different portions 
of its breeding area particular forms occur, differing mainly in 
color and dimensions, but which are all connected together by a 
more or less perfect series of intergradations, and all of which 
may breed together fertilely. Any one at all conversant with 
the geographical distribution of animals or plants knows how 
frequently wide-ranging species are differentiated into a number 
of geographical races, and that the number of such races stands 
usually in a direct ratio to the extent, or diversification, of the 
