GEOGRAPHICAL DISTRIBUTION 365 



France and in the Northern portion of the Arabian Sea. In the case 

 of the Indo-Pacific species of the Arabian sea it is often found that 

 they are absent from the intervening tropical waters, so that their 

 distribution must be explained as occurring at a period when the 

 tropical waters had a more equable temperature. 



(4) In general, the Indo-Pacific region is more probable as the 

 home of the various tropical and subtropical genera and they can be 

 classified into : 



(a) genera with no Atlantic representatives, 



(b) genera with a few Atlantic species, e.g. Haltmeda, Caulerpa, 

 Sargassum^ Dictyota, Scinaia, Galaxaura. 



The following genera are probably of Atlantic origin: Dasy- 

 cladus, Penicillus, Cladocephalus, Batophora. 



(5) Several families in the Laminariales, e.g. Laminariaceae, 

 Alariaceae, are of Boreal Atlantic — Pacific discontinuity. These 

 families must formerly have had a circumarctic distribution but 

 were pushed south by the onset of the Ice Age and then they re- 

 mained in their new habitat when the ice retreated. In this case 

 change of climate in a former epoch provides a satisfactory ex- 

 planation of the present discontinuity. Other genera, however, 

 e.g. Lessonia, Macrocystis, Ecklonia, are of Antipodes-Northern 

 Pacific discontinuity, Macrocystis in particular being primarily 

 circumantarctic, after which it is absent from the tropics, to re- 

 appear again on the Pacific coast of North America and around the 

 shores of South Africa. The two species of the southern hemisphere 

 appear to be identical with the two species in the northern hemi- 

 sphere so that presumably they have disappeared from the inter- 

 vening warm zone. Again, it must be concluded that their migra- 

 tion took place at a time when the temperatures of the ocean 

 waters were more equable, unless it is assumed that the species 

 have since become less tolerant towards temperature. 



Apart from these facts of general distribution there is very little 

 further information in the literature. The Danish workers, 

 Borgesen and Jonsson (1905) and Jonsson (1912), have studied the 

 arctic and subarctic floras in some detail and their results may 

 properly be included here. They concluded that the component 

 species of the flora could be divided into a number of distinct 

 groups : 



