_ JUNE 21, 1901.] 
though individual influence were a wave 
raised by a pebble, and not a permanent 
elevation of level. Individual differences 
could in this view affect the evolution of 
species only when interbreeding is prevented 
by some form of isolation, a supposition 
which has received apparent support from 
the finding of many distinct species in small 
islands or other circumscribed localities. 
That isolation tends to the rapid differentia- 
tion of specific types affords, however, no 
proof that progressive change is due to isola- 
tion, geographical or selectional. That a 
certain peculiarity is manifest in all the in- 
dividuals of an insular or strictly cireum- 
scribed species may mean, not that all the 
individuals are descended from a single 
peculiar ancestor, but that the change tend- 
encies which have originated in this local- 
ity have been confined to it, and have not 
been able to propagate themselves beyond 
the natural barriers.* In the absence of 
such limits specific differentiation might not 
have taken place, but evolutionary progress 
might have been greater by reason of access 
to more varied developmental tendencies. 
Specific differentiation thus affords but 
little indication of the rate or nature of 
evolutionary progress, which often appears 
not to be the result of isolation at all, but 
rather of the aggregation and integration 
of individual variations welcome to the or- 
ganic constitution. In this aspect of the 
subject isolation may be viewed as the ab- 
sence, rather than as the result, of selection. 
* The stability or fixity of type which has been as- 
cribed to small segregations of plants and animals is 
only relative, even under careful artifical selection, 
and from the standpoint of biological history is to be 
interpreted as uniformity rather than as permanence. 
Among the molluscs, which have been supposed to 
furnish examples of great permanence, very diverse 
animals are now known to inhabit shells of extreme 
similarity. Evidently the external skeleton is not an 
adequate index of evolutionary status: in some 
families variation seems to have principally affected 
the shells, in others the softer parts. 
SCIENCE. 
oul 
Species, which have been thought to sup- 
port the opinion that selection brings about 
evolution by inducing various degrees and 
kinds of isolation, can often be much more 
rationally interpreted as instances of the 
manifestation of spontaneous developmental 
tendencies. Groups like the Diplopoda il- 
lustrate the infinity of combinations of char-= 
acters possible without the assistance of 
any of the influences commonly invoked to 
explain the specializations of the more 
adaptive organisms, and thus permit us to 
realize that evolution is one of the causes, 
instead of a result, of biological isolation, 
the effects of which are probably limited to 
specific differentiation. 
To have confidence in organic evolution 
accomplished by selection and isolation in 
the presence of an efficient panmixia would 
require at least double the credulity needed 
to equip the earth from the original pairs 
of the Garden of Eden or of Noah’s Ark, 
since the ancient accounts provided two 
parents for each species while the modern 
begins with one, the offspring of which 
must be placed under such conditions that 
they are prevented from mixing with the 
parental type, either by means of geo- 
graphical separation or through cirecum- 
stances which give the variety immediate 
and exclusive advantage over the older 
forms. The only alternative would involve 
the origin of species from sports so ex- 
tremely divergent that their offspring would 
remain distinct after considerable dilution 
with the parent form. The maintenance 
of such differences through the prepotency 
of new types, in accordance with the 
kinetic view, could not, of course, be in- 
voked under theories accepting panmixia. 
Moreover, it should not be forgotten that 
all segregations of small numbers of in- 
dividuals involve the necessity of close 
breeding, which has been found in domesti- 
cated animals and plants to lead in many 
instances directly toward abnormal and 
