Professor Owen's Address. 385 



To specify or analyze the labours of the individuals who of late 

 years have contributed to advance Zoology by the comprehensive 

 combination of the various kinds of research now felt to be essential 

 to its right progress, would demand a proportion of the present 

 discourse far beyond its proper and allotted limits. Yet I shall 

 not be deemed invidious if I cite one work as eminently exemplary 

 of thespirit and scopeof the investigations needed for the elucidation 

 of any branch of natural history. That work is the monograph of 

 the Chelonian Reptiles (tortoises, terrapenes and turtles) of the 

 United States of America, published last year at Boston, TJ. S., by 

 Prof. Agassiz. 



GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



Observations of the characters of plants have led to the recogni- 

 tion of the natural groups or families of the vegetable kingdom, 

 and to a clear scientific comprehension of that great kingdom of 

 nature. This phase of botanical science gives the power of further 

 and more profitable generalizations, such as those teaching the 

 relations between the particular plants and particular localities. 

 The sum of these relations, forming the geographical distrubutions 

 of plants, rests, peihaps at present necessarily, on an assumption, 

 viz., that each species has been created, or come into being, but once 

 in time and space ; and that its present diffusion in the result of 

 its own law of reproduction, under the diffusive or restrictive in- 

 fluence of external circumstances. These circumstances are chiefly 

 temperature and moisture, dependent on the distance from the 

 source of heat and the obliquity of the sun's rays, modified by 

 altitude above the sea-level, or the degree of rarefaction of the at- 

 mosphere and of the power of the surface to wastefully radiate heat. 

 Both latitude and altitude are further modified by currents of air 

 and ocean, which influence the distribution of the heat they have 

 absorbed. Thus large tracts of dry land produce dry and extreme 

 climates, while large expanses of sea produce humid and equable 

 climates. Agriculture affects the geographical distribution of 

 plants, both directly and indirectly. It diffuses plants over a wider 

 area of equal climate, augments their productiveness, and enlarges 

 the limits of their capacity to support different climatal conditions. 

 Agriculture also effects local modifications of climate. Certain 

 species of plants require more special physical conditions for health ; 

 others more general conditions ; and their extent of diffusion varies 

 accordingly. Thus the plants of temperate climates are more 



