No. 1.] ORGANIC VARIATION. 257 
gressive, but regressive in regard to specialization, in compari- 
son with their ancestral forms. The question before us is, 
then: When in an organ there is at work a progressive 
structural development, simultaneous with a regressive meristic 
development,’ is the organ to be regarded on the whole as 
progressive or regressive ? The answer to this is to be gained 
by determining whether structural complexity is of greater or 
of less morphological importance than is meristic change. 
Now the consensus of opinion among biological investigators 
would show that structural complexity (both chemical and 
histological) is of much greater morphological importance than 
is mere meristic development, — this assumption being, indeed, 
a necessary preliminary in any attempt to homologize different 
organisms. For to pick out an example at random, who would 
venture the opinion that Branchipus occupies a higher morphol- 
ogical position than Astacus, simply on the ground that it 
possesses more numerous extremities ; and would not rather 
conclude that Astacus is the higher organism, because its 
extremities are structurally more differentiated ? Therefore, if 
structural modification is of greater morphological importance 
than numerical (meristic) modification, then when progressive 
structural development is accompanied by regressive numerical 
development, the organ as a whole is to be regarded as develop- 
ing progressively ; and conversely, when regressive structural 
development is simultaneous with progressive numerical de- 
velopment, the course of development of the organ is to be 
considered regressive. 
It still remains to accentuate an apparent law, which 
is generally recognized, with reference to this frequent con- 
comitance of numerical and structural development. Appar- 
ently a progressive structural development frequently causes 
a regressive numerical development, as we find when, by 
the coalescence of previously separate meristic organs (2.e. 
through their regressive numerical development), a com- 
pound organ is produced, which is higher morphologically 
than was any one of the previously separate organs. How- 
ever, a numerical reduction of the units of a meristic series 
can proceed, and perhaps does so more usually, without 
