. Ontario Sessional Papers, 1905, . in-creasing heat by ascending the mountains. Where these were high enoughto furnish proper climatic conditions for the colony thus formed, communi-ties have been carried through to the present day, and the collector may findon the summit of the highest Rockies the identical species that he catches onthe frozen shores of Labrador, though the intervening valleys of the UnitedStates and southern Canada will not furnish him a single specimen. Another well known characteristic of insect life, is the tendency of suc-cessful species—those which are in such
. Ontario Sessional Papers, 1905, . in-creasing heat by ascending the mountains. Where these were high enoughto furnish proper climatic conditions for the colony thus formed, communi-ties have been carried through to the present day, and the collector may findon the summit of the highest Rockies the identical species that he catches onthe frozen shores of Labrador, though the intervening valleys of the UnitedStates and southern Canada will not furnish him a single specimen. Another well known characteristic of insect life, is the tendency of suc-cessful species—those which are in such perfect harmony with their sur-roundings that the increase in number is very rapid—to spread, and to in-vade new territory. Where this spread is natural, that is to say, not depend-PDt on human agency, it usually takes place along definite lines. These linesaie determined by various conditions, depending on the nature ot the insect,as well as on the country which it is invading. Some are very sensitive to 1904 ENTOMOLOGICAL SOCIETY. 43. MAf SHOhr//\/& JD/S 7n/Bur/onJOF /. C. /set/e/osen/Z/s. fl_ C. CcAo- 3. ,. /t^ />m,f o/ f/^ Greai JBo-s^n. Fig. S. Maj. ut till- (.ircat 44 THE REPORT OF THE No. 1^ climatic changes, and are limited in their distribution by the isothermal linesor by the extent of rainfall. Thus, a desert may prove impassable for onespecies, while another may be equally unable to surmount the barrier pre-sented by a low swampy tract, a high mountain range or a great plateau,l^he valley of a large river, with its gradual slope and protected nooks, formsa natural highway for the passage of migrating or spreading species. Within the Great Basin of North America, certain problems of insect dis-tribution are presented in almost perfect purity. In these desolate areasaie spots rarely trodden by the foot of man, still supporting the life that hasbeen theirs since the time when the valleys were tilled with lakes that haveleft thei
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