Chapter XVII 



GEOGRAPHICAL DISTRIBUTION, 

 LIFE FORM 



Many of the studies of algal distribution are based on a considera- 

 tion of continuous or discontinuous distribution which are, for 

 convenience, discussed as though they were separate phenomena, 

 although it is clear that no distribution can be absolutely continu- 

 ous. When, however, it is found that an area in which the locaUties 

 are fairly close together is separated by the width of a continent or 

 of an ocean from another similar area, then we may talk of dis- 

 continuous distribution. The problem is rendered more difficult by 

 the unreHability of earHer records and the somewhat scanty litera- 

 ture, especially for tropical and sub-tropical areas. The few studies 

 (Svedelius, 1924; Borgesen, 1934), that have been published have 

 estabHshed certain general features which are briefly summarized 

 below : 



(i) There is a general resemblance between the algal floras of 

 the West Indies and the Indo-Pacific. Vicarious pairs of species 

 (two separate species closely related morphologically and yet widely 

 separated geographically) are known and even vicarious generic 

 groups. Species in the genus Hormothamnion in the Myxophyceae 

 have a Caribbean-Indo-Pacific discontinuity, whilst there are 

 several vicarious pairs in the genus Neomeris (Fig. 226). The ex- 

 planation of these discontinuities which has been advanced by 

 Murray, namely change of climate in former epochs, would only 

 appear to explain certain cases, e.g. certain species in the Lamin- 

 ariaceae (cf. below), whilst it is equally obvious that the factors 

 operating at present do not provide an adequate explanation. The 

 only feasible hypothesis would be to postulate migration during an 

 earlier epoch when there was a sea passage through the Panama 

 isthmus, and this involves a migration not later than the Cretaceous. 



(2) There are some species which are common only to the 

 Western Atlantic and the western part of the Indian Ocean around 

 Madagascar, e.g. Chamaedorus peniculum and three species of 



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