June 22, 1917] 



SCIENCE 



641 



ically forced ascent of the air again brings it 

 to the condensation point, and at 8,000 feet, 

 rain may come in summer. The rainfall on 

 the windward side of the coast range occurs 

 in winter, for then the land is more nearly the 

 temperature of the water. In summer, even 

 with the sea-breeze twice as strong, the heated 

 land surface warms the ascending air too 

 rapidly for the occurrence of precipitation. 

 As little moisture is lost in summer on the 

 coast range, the relative humidity is higher in 

 the interior, so that as the strong winds drive 

 up the mountains, there is more moisture avail- 

 able for precipitation. With unusual wind fre- 

 quency from the north, there are heavy coast 

 rains at intervals of four to ten years, for then 

 the cold coast water is replaced by a warmer 

 current. 



Throughout Peru, the trade winds are in 

 control : there is the wet windward slope, the 

 semi-arid interior plateau and the arid leeward 

 coast. The extraordinary inner diversity of 

 climate in the eastern mountains is due to the 

 difference in exposure to the trade winds, and 

 to the diilerences in altitude. The contrasts 

 within the desert coast region are the result of 

 the effect of the topography on the daily sea- 

 breezes of varying strength blowing off the 

 cold Humboldt current. 



Chaeles F. Brooks 



Tale Universitt 



SPECIAL ARTICLES 



THE DISTRIBUTION OF ENDEMIC SPECIES IN 

 NEW ZEALAND 



The flora of ISTew Zealand is so exceptionally 

 rich in endemic forms that its study promises 

 interesting results if taken up under the mod- 

 ern conceptions concerning the origin of 

 species. It embraces among the angiosperms 

 902 endemics confined to New Zealand proper 

 against 399 forms of wider distribution, 98 of 

 which are confined to New Zealand and out- 

 lying islands. This consideration has induced 

 J. C. "Willis to make a statistical study of this 

 flora^ and to compare it with the results de- 



iDr. J. C. Willis, "The Distribution of Species 

 in New Zealand," Annals of Botany, Vol. 30, No. 

 119, July, 1916. 



duced from his similar treatment of the en- 

 demic forms of Ceylon.^ 



Willis has proposed a new principle for ex- 

 plaining the distribution of plants in general, 

 which is that " the area occupied by any given 

 species (taken in groups of twenty or so) at 

 any given time in any given country in which 

 there occur no well-marked barriers depends 

 upon the age of that species in that country." 

 This proposition he calls his hypothesis of 

 " age and area." It is intended to convey the 

 idea that adaptation, although it may be of 

 use in determining the frequency of a species 

 within its area, is not in general a factor of 

 wider operation. No results show in the fig- 

 ures which can be attributed to it. Excep- 

 tional instances, where this seems to be the 

 case, are almost always the result of changes 

 in environic conditions, made by man, and it 

 is a well-known fact that such rapidly spread- 

 ing forms invade a country along its roads 

 and railroads, occupying chiefly waste fields 

 and stirred-up soil. 



If, however, we leave these out of consid- 

 eration and concern ourselves only with ordi- 

 nary wild species, statistical study seems the 

 only means to get average and reliable results. 

 Taken in small groups of, e. g., 10-20 species 

 these statistical results prove to be the same 

 everywhere and in all the larger families. A 

 general cause must govern this phenomenon, a 

 cause which is, at any rate, independent of 

 morphological and biological qualities. 



New Zealand is very convenient for deter- 

 mining the area of its species, for the islands 

 are spread out in a long curve running in gen- 

 eral north and south for about 1,080 miles, 

 with an average breadth of 100. Therefore 

 longitudinal range can simply be substituted 

 for area and this can be determined by divid- 

 ing the country by transverse lines at every 

 twenty miles. Moreover, it consists chiefly of 

 two parts : North Island and South Island, 

 which do not show any essential barriers to 

 the spreading of plants, but are separated by 

 a channel broad enough not to be passed by 



2 See the review in Science, N. S., Vol. 43, No. 

 1118, pp. 785-787, June, 1916. 



