Evolution / by Patrick Geddes and J. Arthur Thomson.

  • Patrick Geddes
Date:
[1911?]
Catalogue details

Licence: Public Domain Mark

Credit: Evolution / by Patrick Geddes and J. Arthur Thomson. Source: Wellcome Collection.

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    germinating on ground already occupied and from animals. The amount of food, of course, gives the extreme limit of numbers; very frequently, however, the check is given by the attacks of enemies, as on game by 44 vermin.” Changes of climate play an im¬ portant part, and periodical seasons of extreme cold have destroyed as many as four-fifths of the birds of an observed area. Epidemics, too, may occur. In many a species a large stock of individuals is often essential to its continuance. Complex and unexpected checks and re¬ lations exist between organic beings which have to struggle together; witness the pro¬ found alteration of the flora and fauna of a heath when planted with Scots pine, these again being wholly dependent upon the ex¬ clusion of cattle. But in several parts of the world insects determine the existence of cattle. Again, red clover depends for fertil¬ ization upon the humble-bees, these upon immunity from the attacks of field-mice, and thus indirectly upon the number of cats. Hence no bees, no clover, and the more cats, the more clover. The struggle will almost invariably be most severe between the individuals of the same species, for they frequent the same districts.