INTRODUCTION. 9 



of Yucca and Aloe in their respective continents are to be sought in 

 the operation of the environment, and the spread of an introduced 

 plant in a new continent must be controlled by environmental con- 

 ditions just as it was controlled in its native continent. 



Evolutionary activities may be thought of as supplying the raw 

 materials out of which the physical environment has made the present 

 distributional complex of the earth's surface. We can not hope at 

 present to understand why a strong development of the genus Yucca 

 has occurred in the southwestern United States and a rich develop- 

 ment of Aloe in South Africa, and the answer to such a question, if 

 forthcoming, would have only a remote relation to the study of the 

 present influences which are limiting the distribution of the members 

 of these two stocks in their respective continents. 



The role of paleochmatic factors is also an immanent one in the 

 determination of plant distribution. Many species have a known 

 ancestry and a known ancestral range, at the same time that they now 

 inhabit areas of such small size that it is difficult to beheve that any 

 present physical conditions are restricting them. The history of these 

 plants has been one of extinction and retraction, due to changes in 

 their old extended environment. They are now unable to regain their 

 old ranges, or even to spread over relatively short distances. These 

 species may well be looked upon as physiologically and genetically 

 decadent, and either decadent because they are geologically old or 

 else old because they have failed to change their requirements to that 

 shght extent which might permit a greater extension, or to that greater 

 extent that would have reacted upon form in such a manner as to make 

 us regard the resulting plants as distinct species from the old ones. 



In addition to the old and restricted, or relict, species we have 

 another class of plants of limited range, those which are apparently 

 just appearing on the scene and have not yet had time to occupy the 

 entire area in which they might be expected to find congenial condi- 

 tions. These plants are generally members of large genera, such as 

 Opuntia, Antennaria, and Cratoegus, or else of genera which have been 

 recently subdivided, whereas the relict species are usually members 

 of small genera. It is not always easy to decide whether a given 

 restricted distribution is of the relict type or of the ''novitiate" type, 

 as we may call the emerging species. Our decision, in the lack of other 

 evidence, is apt to be based upon the existence or absence of fossil 

 records of the species or its alHes, or else upon the size and phylogenetic 

 position of the genus or family. Such a restricted conifer as Pinus 

 mayriana may well belong in either class, and the members of many 

 small or monotypic genera of Mexican Compositse may also stand in 

 a doubtful position. 



A very considerable number of our plants belong neither to the relict 

 nor to the novitiate class. They are of such age that they have had 



