150 TEST CASES [ch. xiii 



edges, so that when one draws a Hne round the outermost locaUties 

 of each species one obtains a picture not unlike that which is 

 called a contour map by the geographers, such as may be seen in 

 any good guide-book to hilly country. If the genus be small, 

 there will probably be only one generic centre, whilst the larger 

 that it becomes, the more broken will the central part be, with, 

 more and more regions in which there is a concentration of 

 species, like regions of the higher peaks in a geographical contour 

 map. So long as a genus is of small or moderate size, the outer- 

 most or boundary species seems usually to be one species only, 

 but as it grows larger it becomes rarer for there to be one species 

 occupying the whole generic area, and one begins to find local 

 concentrations of species in widely separated parts of the world, 

 like that which is shown here in the map of New Zealand, with 

 the species of Ranunculus there found. Here one finds three 

 "wides" (as I have called the species which have a dispersal 

 outside the country in question) occupying the whole area of the 

 islands of New Zealand, and also reaching eastwards to the 

 Chatham Islands, 375 miles away. Their distribution is thus by 

 far larger than that of any other buttercups in New Zealand 

 (fig. 9). The fourth wide has a distribution not very much less 

 than that of the most widely dispersed endemic. The total length 

 of the islands is 1080 miles and the breadth does not vary very 

 much from 100 miles, so that the longitudinal range may be taken 

 as a reasonable measure of the dispersal of a species. The en- 

 demics are evidently crowded together rather south of the middle 

 of the South Island, whilst they fade out completely before the 

 north end of the North Island is reached. Of the twenty-eight 

 endemics, ten have a range not exceeding 60 miles of the length 

 of New Zealand. If one take the ranges in differences of 200 miles 

 — 200, 400, etc. — one finds that fourteen, seven, five, one, one 

 species have these ranges, or, in other words, the figures form the 

 usual hollow curve of distribution, and this is shown by any New 

 Zealand concentration of the larger genera. The general impres- 

 sion that one gains from a map like this is that the genus 

 Ranunculus entered New Zealand probably from the south, and 

 at some place in the southern half of the South Island, where the 

 incoming species began giving rise to endemics, and on the average 

 each species, wide or endemic, spread to the distance allowed by 

 its age, and suitability to the conditions with which it met. 



The same type of contour distribution is shown by the genera 

 of a family, as fig. 10 shows. Incidentally, these contour maps 



