258 MONTGOMERY. [Vor. XII. 
coalescence ; and such a regressive numerical development 
is also usually associated with a progressive structural 
development of those units of the meristic series which are 
retained. Examples of such cases may be found in abundance, 
and it is sufficient here to refer to the lateral line of sense- 
organs in the Vertebrates, where a progressive development of 
certain of the individual organs is concomitant with a reduction 
in the number of the units comprising the series. And witha 
view to many analogical cases, which it is unnecessary to give 
in detail here, since any zoologist may recall many to mind, 
the law will be found to be of general application, that pro- 
gressive structural development is furthered by regressive 
numerical development, since in this way greater centralization 
of the forces of growth would ensue. And although regressive 
structural is sometimes concomitant with regressive numerical 
development, as exemplified in the case of certain parapodia of 
sedentary Annelids, I recall no case of the concomitance of 
progressive structural and progressive numerical development ; 
but I would not imply by this that such a concomitance can- 
not or does not occur, but rather that such a concomitance is 
of so infrequent occurrence as not to render invalid the 
general law just mentioned. My reason for accentuating this 
law of the usual concomitance of progressive structural with 
regressive numerical development is in order (1) to characterize 
concisely a relation, which, although already recognized, has 
not yet been awarded much attention from zodlogists ; and (2) 
to emphasize a point which may serve as a criterion of pro- 
gressive development. 
By the term development is meant here any organic process 
of change acting in the organism; when such a change tends to 
further complicate the structure, it has been termed progres- 
sive development (evolution, Hxtwzckelung); when it tends to 
simplify the structure, regressive development (degeneration, 
Rickbildung). Speaking generally, development leads towards 
(1) the production of new species, or (2) towards the extinction 
of already present species ; obviously, development cannot be 
conceived as holding a species stable (z.e. unchanging), since 
development always implies an organic change. 
