ROUTES OF MIGRATION, ETC. 
393 
.simplicity of their various parts and contiguity to the evolutionary 
area, the most simply organized being the furthest removed from the 
evolutionary centre and leaving fewest traces of their former presence 
therein, while in accord with increasing complexity and closer affinity 
to the dominant race are the occupied districts more and more closely 
adjacent thereto. 
Although the general direction of this migration of .species is 
nndeniable, yet the most closely allied forms, from the similarity of 
their foods and modes of life, being naturally the most keenly 
competitive, tend to disperse in diverse directions, where less severe 
opposition than that of their closest allies is encountered, and we 
have in this also the explanation of the different local areas often 
occupied by closely related forms and the eventual restriction of their 
general range; thus Helix hortensis tends to be more northern than 
Helix nemomlis, and Helix aspersa more western than Helix pomatia, 
and although these species are undoubtedly sometimes found as- 
sociated, this association is not a permanent one, as the weaker .species 
must in the end eventually succumb. 
There has been little mutual interchange of faunas, as has been 
so often affirmed, the observed intermingling being essentially due 
to the invasion of weaker areas by the stronger forms of life, and 
not to the occupation by the weaker of districts already tenanted by 
vigorous species, although a few primitive forms, owing to their small 
sfze, the adoption of special foods, or modes of life le.ss actively com- 
petitive with the dominant races, may obtain or have retained a footing- 
in districts from which most of their congeners have long been expelled. 
M. Bourguignat’s affirmation that mollusks can only be succe.ssfully 
acclimatized from North to South or from East to West had doubtless 
reference to his own observations in France, and in this case exactly 
expresses the direction in which the introduced mollnsk would be 
placed amongst weaker races, and tlms have more likelihood of pros- 
pering, at least so far as competition with indigenous forms was 
concerned ; this principle is clearly demonstrated by the rapid dif- 
fusion of a relatively strong .species when placed amongst a much 
weaker fauna, as evidenced by the sparrow and other forms of life in 
North America, the rabbit, etc., in Australia and New Zealand, the 
successful establishment of Helix aspersa at so many points of the 
globe, and by many other instances of the rapid increase of stronger 
races when jilaced in the midst of a palpably weaker fauna. 
