ILLUSTRATIONS (13). PHANEROGAMIC PLANTS. 291 



Lepidoptera wanting; while the number of Phanerogamia 

 found there scarcely exceeds 1000. The Insect-fauna of Great 

 Britain is estimated at 11,600. Such a preponderance of 

 animal forms will appear less surprising when we remember 

 that several of the large classes of insects live only on animal 

 substances, whilst others subsist on agamic plants (Fungi), 

 and even on those which are subterranean. Bombyx Pini, 

 the Pine Spider, the most destructive of all forest-insects, 

 is infested, according to Ratzeburg, by no less than thirty-five 

 parasitical Ichneumonidae. 



These considerations have led us to the proportion borne 

 by the number of species growing in gardens to the gross 

 number of those already described and preserved in herba- 

 riums ; it now remains for us to consider the proportion of 

 the latter to the conjectural number of species existing on 

 the whole earth, or, in other words, to test their minimum 

 by the relative numbers of the different families i. e. by 

 variable multipla. A test of this kind gives, however, such 

 low results for the lower amount, as plainly to show that even 

 in the large families, which appear to have been the most 

 strikingly enriched in recent times by the researches of descrip- 

 tive botanists, our knowledge is still limited to a very small 

 portion of the treasure actually existing. The Repertorium of 

 Walpers which completes Decandolle's Prodromus of 1825 to 

 1846, gives 8068 species of the family of the Leguminosae. 

 We may assume the mean ratio to be -^5 since it is -^ 

 in the tropical zone, T J T in the middle temperate zone, and 

 -$ in the cold northern zone. The described Legurninosae 

 would therefore only lead us to assume that there were 

 169,400 species of Phanerogamia existing on the earth, 

 whereas the Compositae, as already shewn, testify to the 

 existence of more than 160,000 known Phanerogamia, i. e. 

 such as have been described or are contained in herbariums. 

 This discrepancy is instructive, and will be further elucidated 

 by the following analogous considerations. 



The larger number of the Compositse, of which Linnaeus 

 knew only 785 species, and which have now increased 

 to 12,000, appear to belong to the Old Continent. At 

 least Decandolle described only 3590 American, while he 

 estimated the European, Asiatic, and African species at 

 5093. This abundance of Compositae in our vegetable 



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