HOW AND WHENCE? 177 
of the barriers they would have been called on to 
cross. An investigation on these lines would be 
lengthy, and out of place here. The reader has 
already from Chapter III. acquired some insight into 
the powers as well as the limitations possessed by 
seeds for crossing such barriers. Summing up the 
evidence briefly, it may be said that the seeds of 
none of the southern group float in water; conse- 
quently transport by currents is ruled out. Secondly, 
none of them is so light (see pp. 62-69) as to render 
it possible for them to cross the intervening sea by 
wind currents; very much the lightest seeds in the 
group are those of the Orchid Neotinea intacta, yet 
even these could not on any reasonable theory have 
been transported by wind from the plant’s nearest 
station (in Southern France); the high speed of fall 
of the small seeds of the Pyrenean Heaths or Saxi- 
frages renders their wind transport, even from the 
smaller distance which has to be reckoned with, in 
their case still more improbable. There is left, then, 
the agency of birds (see p. 70): can we look to these 
swift messengers for assistance? The rapid diges- 
tion of birds renders it futile to expect that even those 
which do not crush the seeds which they eat could 
bring over from the Pyrenees seeds which they have 
swallowed; so we are forced back on the uncertain 
method of ectozoic dispersal: that is, on the assump- 
tion that seeds of these plants have been imported by 
becoming entangled in the feathers of birds, or by 
adhering—possibly with the aid of mud—to their 
feet. That seeds are transported by these means has 
been shown by the observations of Darwin and other 
observers; but that the seeds of a number of different 
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