CH. xiii] D. GEOGRAPHICAL DISTRIBUTION 157 



so on. There are about forty well-separated genera, with well- 

 marked characters of flower and fruit, as well as strongly marked 

 structural characters of the vegetative body. It is impossible to 

 suppose that such structural characters as a bi- or uni-locular 

 ovary can matter in the struggle for existence to a family like 

 this, whose life is passed under water, the flowers only appearing 

 a few days before their final death. The flowers in Asia, and to a 

 considerable extent elsewhere, are fertilised by wind, so that 

 their structural features are even less important to them than 

 usual, though they mostly show the extreme of zygomorphism 

 and stand rigidly vertical. The fruits produce a vast mass of seed, 

 among which perhaps one in ten thousand may produce a new 

 plant. The seeds have no adaptation for clinging to the rock, so 

 that the survivors must be determined by chance. 



A great many other families also show great variety in form 

 though living in conditions that are comparatively uniform. 

 Larger families are in general found to be living in a greater 

 variety of conditions than small, but there are no general rules. 

 But to co-ordinate the number and variety of the genera and 

 species with the variety of the conditions was always an in- 

 soluble problem until it was shown that mere age had a great 

 deal to do with it. With few exceptions, the older a family was, 

 the greater the variety of conditions that it occupied, but there 

 was no arithmetical relation between the two. 



We may take a few examples of families and genera that show 

 a considerable variety in themselves, without occupying a cor- 

 responding variety of conditions. Pandanus, which is found 

 almost entirely in the uniform conditions of seashores or marshes, 

 has 180 species. The Naiadaceae (1 genus with 35 species) and the 

 Aponogetonaceae (1/25) are water-plants of very uniform condi- 

 tions. The Cyperaceae, mostly in swamps or in sandy places, both 

 of which must be very uniform, show 85/2600. The Bromeliaceae, 

 epiphytic or on rocks, and therefore in very uniform conditions, 

 are 65/850, the Juncaceae, in damp and cold places, 8/300. The 

 orchids, largely epiphytic, where the conditions must be very 

 uniform, are 450/7500. The Salicaceae, mostly mesophytic trees, 

 are 2/180, the Loranthaceae, woody semi-parasites, are 30/520. 

 The Balanophoraceae, internal parasites, whose conditions must 

 be very uniform, show 15/40, and Orobanche, a semi-parasite, has 

 90 species. The halophytic Chenopodiaceae have 75/500, the 

 xerophytic Aizoaceae 20/650, the water-inhabiting Nymphaea- 

 ceae 8/50, the insectivorous marsh-loving Drosera has 90 species. 



