114 



SCIENCE 



[Vol. LV, No. 1414 



hybrid blackberries, spreading readily by pros- 

 trate branches that root at the tip, may easily 

 establish themselves in extensive growths. In a 

 recent classification of the blackberries of 

 New England Brainerd and Peiterson isolate 

 23 hybrid species of the 12 primary species 

 that are recognized, and they give an addi- 

 tional list of 32 suspected hybrids. Violets do 

 not spread so prolifically as brambles but 

 there are a number of hybrids known which 

 maintain themselves in Nature by vegetative 

 gi-owths. Other groups of plants readily 

 propagating from stems are likely to show 

 similar proportions of impure species as they 

 are more thoroughly studied. 



With the data before us on the widespread 

 occm-rence in Nature of impure species we 

 wonder what will be the reaction of systematic 

 botany. It will be impossible for the manuals 

 to include the many hundreds of lines which 

 the geneticist may isolate as impure species 

 although they may be definite units of floras. 

 There will be little satisfaction in attempts to 

 identify in the field races which can only be 

 established by experimental studies of the 

 garden. Are these impure species to be 

 grouped for convenience as collective species 

 regardless of their- true positions and relation- 

 ships? Truly the paths of the systematist and 

 ecologist have not been made easier by the 

 progi'ess of genetics. 



Bradley Mooee Davis 

 Univeesitt of Michigan, 



THE TREND OF EARTH HISTORY' 

 II 



Through the millions of years represented 

 by the Tertiary period the mammals differen- 

 tiated slowly along the conventional lines 

 which had been previously marked out in large 

 measure by the reptiles. Some became adapted 

 to life on the dry plains, others in the forested 

 river flats, others in the high mountains, the 

 tree-tops and the tropical jungles. A few of 

 them learned to fly more or less successfully, 

 some burrowed under ground and stiU others 

 became aquatic. In a general way they did 

 what the various kinds of reptiles had done 

 before them in the Mesozoic era, but, on the 

 whole, they seem to have done it better. 



Finally, about the end of the Tertiary period 

 or later, the next great advance was made by 

 the genus Romo — an offshoot of one of the 

 most insignificant groups of mammals. In 

 consequence of this achievement, the entire 

 gi'oup has been dignified with the name of 

 Primates. From this oft'shoot so many sur- 

 prising things have developed that it is hard 

 to say which one was fundamental. Undoubt- 

 edly, one of the first new habits of the human 

 genus was the use of tools. We may reason- 

 ably suppose that only one of the less spe- 

 cialized types of mammals, a creature pos- 

 sessing flexible flngers and hence the power lo 

 grasp a stone or a club in the hand, coulJ 

 acquire such ability. Possibly it was this 

 initial power that gave the flrst impetus to the 

 higher progress of the pre-human stock. Be 

 that as it may, the progress of the human race 

 seems to have depended largely on the ability to 

 invent and use other things, such as fur-cover- 

 ed skins for clothing, the spear and bow-and-ar- 

 row for the chase, the fish hook, the needle, tlie 

 potter's wheel and so on through the long list of 

 human contrivances. As Bergson has remarked, 

 each human tool and machine serves as a new 

 and additional bodily organ and so multiplies 

 our functional activities to a wonderful degree. 

 The development of higher intelligence went 

 on side by side with this multiplication of 

 inventions, doubtless, on the one hand, being 

 stimulated by it and, on the other, making 

 possible its continuation. 



Looking back over the great contributions 

 which the various animal groups have devised 

 and elaborated in the vast stretches of geologic 

 time, and omitting only that of the human 

 race — which is too new to be impartially 

 judged — it will be observed that, although each 

 of these innovations has brought temporary 

 success and domination to its holders, it has 

 never been able to insure the permanency of 

 the exa,lted position so attained. Experimenta- 

 tion seems to be nature's endless pastime. Her 

 appetite for it is insatiable; and, no matter 

 how interesting the results of the trials already 

 made, there are always more to come. As John 

 Burroughs once said, "Nature hits the mark, 

 because she shoots in all directions." 



In that part of the history of man which is 

 sufficiently well known, we perceive a series of 



