154 TEST CASES [ch. xiii 



conditions in Britain are perhaps more varied than in any part of 

 Europe, yet no genus has the centre of its map there, and several 

 hundred genera have the one marginal species in Britain, and that 

 only. If this conception were correct, the variety of conditions 

 would tend to increase away from the sea. If one take family 

 contours such as those shown in the map of Menispermaceae on 

 p. 152, the case is even better marked. Such families as Umbelli- 

 ferae or Cruciferae have their centres of aggregation well marked 

 in the Eastern Mediterranean and Central Asia. But not only do 

 they show there the maximum of species in general, but also the 

 maximum number of monotypic genera with one species only. 

 These are usually set down as relics, and why should relics be 

 most numerous at headquarters? In their anxiety to prove the 

 validity of natural selection people have worked upon more or 

 less independent lines, which often clash badly with one another. 

 Workers with floras of islands or of mountain chains have urged 

 the conception of endemic species and monotypic genera as relics, 

 regardless of the fact that other workers have shown that these 

 relics are most abundant at the "headquarters" of the family, 

 and are regarded as showing the great suitability of the family to 

 that particular region. 



A very difficult problem for supporters of the idea that condi- 

 tions and their variety have anything to do with the contours is 

 provided by their behaviour in New Zealand. The northern inva- 

 sion of plants shows contours beginning in the north, with the 

 last species of the genus somewhere in the south. The southern 

 invasion begins in the south, and its contours fade away to the 

 north, but each invasion passes over the centre of the other 

 (where the conditions are supposed to be so varied) without 

 taking the least notice of it. There cannot be conditions that only 

 affect northern plants, or only southern, as the case may be. 



TEST CASE XXVIII. TAXONOMIC RESEMBLANCES OF 

 (GEOGRAPHICALLY) WIDELY SEPARATED PLANTS 



This case has already been published, and the following descrip- 

 tion is largely quoted, by kind permission of the Linnean 

 Society, from a paper in their Proceedings of 21 April 1938: 



Whilst when first published both these conceptions — Differen- 

 tiation and Young Beginners^ — were much opposed to current 

 beliefs, there is no doubt that the latter, at any rate, is gaining 



1 I.e. the conception that the bulk of the very local endemic species, 

 especially in warmer countries, are young species starting life. 



