256 MONTGOMERY. [-VoL. XIT. 
series; while regressive. development (degeneration, Réck- 
bildung) tends to simplify the structure, to change the position 
and dimensions, and (subject to certain limitations) to decrease 
its number in a meristic series. Since either process may pro- 
duce a change of position of the organ involved, such a change 
furnishes no criterion whereby to judge the kind of develop- 
ment, until it can be determined whether the direction or 
amount of change of position differs according as the mode of 
development is progressive or regressive. And whether change 
of the dimensions gives us a criterion of the kind of develop- 
ment is also doubtful; although, since it is the general rule 
that increasing complexity of structure is usually accompanied 
by actual increase in size, it might be concluded that an 
increase in size of an organ denotes frequently the action of 
progressive development. But since it is only a general rule, 
and by no means a law without exceptions, that increase in 
size goes hand in hand with increasing structural complexity, 
it would be safer to eliminate change of dimension from our 
criteria for distinguishing between the two modes of organic 
development. 
We find, however, a certain criterion for estimating the kind 
of development, in change of structure; for an increasing com- 
plexity of structure is distinctive of progressive development, 
while, on the other hand, a decreasing complexity is the sign 
of regressive development. Further, an increase in the num- 
ber of meristically (segmentally or metamerically) arranged 
organs is a criterion of progressive development, as a decrease 
in the number of similar organs is of regressive development, — 
provided that, in each case, no structural changes are simul- 
taneously taking place. This standpoint will hardly be dis- 
puted, for we consider an organism A to be morphologically 
higher than an organism #, when A possesses a larger number 
of organs in a given meristic series than does 4, even though 
these organs are otherwise structurally equivalent in A and BZ. 
But it is necessary to consider the case, when a progressive 
meristic development is acting together with a regressive 
structural development, or vice versa: as e.g. in the carpus of 
the Ichthyosauria, where the phalanges are meristically pro- 
