262 MORPHOLOGY OF ANGIOSPERMS 



stage In a long history of distribution, and a knowledge of this 

 history is an essential factor in any explanation of the present 

 distribution. Unfortunately, very little of this history is avail- 

 able, and this presentation must content itself with indicating 

 the present relation of groups to the earth's surface, without 

 any attempt at explanation. This is particularly unfortunate, 

 since a lack of historical evidence may vitiate many conclu- 

 sions. If this lack of historical testimony be added to the 

 lack of any adequate record of the geographic distribution of 

 existing species, it becomes evident that the generalizations pro- 

 posed must be of the most tentative character. With this ex- 

 planation the following statements may be given their proper 

 weight. 



MONOCOTYLEDONS 



It is possible to present the distribution of the ten alliances 

 of Engler in the order of their supposed relationship, a method 

 that may be of service in the subsequent consideration of the 

 ancient history and phylogeny of the group. One genetic group 

 is supposed to include the three following alliances. 



PANDANALES. The Pandanaceae (screw-pines), apparently 

 the most primitive of Monocotyledons, belong to the general 

 region of the Indian Ocean. Associated with them in relation- 

 ship are the Typhaceae, found in aquatic conditions throughout 

 the world, but most abundant in the tropics ; and the Spargania- 

 ceae, restricted to the temperate and boreal regions of the 

 northern hemisphere and also of the Australasian region, and 

 not represented in the tropics. The series as a whole shows 

 wide adaptations to temperature, but not to soil conditions, with 

 the primitive forms massed in the oriental tropics. 



PALMALES. The Palmaceae are about equally divided be- 

 tween the oriental and occidental tropics, with no temperate 

 outliers, but not a species or a genus is common to the two 

 hemispheres. The geographical association of the palms and 

 screw-pines in the orient is in favor of their supposed relation- 

 ship, but the palms of the Occident need explanation, especially 

 since Phytelephas, regarded as a genus intermediate between 

 Pandanaceae and Palmaceae, is an American genus. The pres- 

 ent distribution of palms is an excellent illustration of the de- 

 velopment of continental diversities, which in this case has 



