
Igor] BRIEFER ARTICLES 429 
closely related to opportunism mentioned above. This means that 
when, through a change in some condition of the environment, the 
necessity arises for the performance of a new function, it will be 
assumed by the part which happens at the moment to be most avail- 
able for that purpose, regardless of its morphological nature, either 
because that part happens to have already a structure most nearly 
answering to the demands of the new function, or because it happens 
to be set free from its former function by change of habit, or because of 
some other non-morphological reason. It is due to the operation of 
this principle that structures of the most different morphological origin 
may come to serve the same function, and correlatively, structures of 
the same morphological origin may come to serve the most different 
uses. The genus Pereskia, in the Cactaceae, includes mesophytic 
climbers with true broad leaves, from which all gradations in reduction 
of leaves and condensation of stem may be traced even to the typical 
desert forms of Cereus. Now, one division of Cereus returns to a 
life in the woods, where the demand for an increase of green surface 
is felt; no attempt, however, is made to restore the old leaves (now 
reduced to tiny scales), but the stem enlarges and branches, while the 
vertical ribs, developed during the desert habit, are expanded farther- 
and made to function as leaves. As the mesophytic habit becomes 
more extreme, the ribs become larger in size and fewer fn number 
until finally, in Phyllocactus, but two remain, and these become so 
flattened and arranged in such a manner on the branch that they form 
a physiological and anatomical leaf. Here we have a case, indubitable 
because abundant intermediate steps persist, in which a physiological 
leaf has been developed from a morphological stem, purely by follow- 
ing the line of easiest accomplishment, or least resistance, at the 
moment; no single step is in itself remarkable, but the sum total 
yields a very remarkable result. This principle is, of course, equally 
applicable to both systems of morphology. 
SIXTH, the principle of metamorphosis by transformation, as con- 
trasted with metamorphosis by differentiation, which means that when, 
in response to any influence, a new function and hence structure 
(function-structure) is assumed by any part, this always comes about, 
both ontogenetically and phylogenetically, through the transformation 
or alteration of a previously existing function-structure in that part, 
and never through the differentiation of a new function-structure out 
_ Of a previously functionally-indifferent or unspecialized structure. In 
