560 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



the way was opened for an infinite series of adaptations to varying 

 environments, entailing structural modifications in which the 

 enhanced plasticity of the new type of animal found expression. 



Nature tried innumerable experiments with the new type of brain 

 almost as soon as the humble Therapsid-like mammal felt the impe- 

 tus of its new-found power of adaptation. In turn the Prototherian 

 and Metatherian types of brain were tried before the more adaptable 

 scheme of the Eutherian brain was evolved. 



Tlie new breed of intelligent creatures rapidly spread from their 

 South African home throughout the whole world and exploited every 

 mode of livelihood. The power of adaptation to the particular 

 kind of life each group chose to pursue soon came to be expressed 

 in a bewildering variety of specializations in structure, some for 

 living on the earth or burrowing in it, others for living in trees or 

 even for flight; others, again, for an aquatic existence. Some 

 mammals became fleet of foot and developed Hmbs specially adapted 

 to enhance their powers of rapid movement. They attained an 

 early preeminence and were able to grow to large dimensions in 

 the slow-moving world at the dawn of the age of mammals. Others 

 developed limbs specially adapted for swift attack and habits of 

 stealth successfully to prey upon their defenseless relatives. 



Most of these groups attained the immediate success that often 

 follows upon early speciaUzation, but they also paid the inevitable 

 penalty. They became definitely committed to one particular 

 kind of hfe, and in so doing they had sacrificed their primitive 

 simplicity and plasticity of structure and in great measure their 

 adaptabiUty to new conditions. The retention of primitive char- 

 acters, wliich so many writers upon biological subjects, and espe- 

 cially upon anthropology, assume to be a sign of degradation, is not 

 really an indication of lowHness. We should rather look upon high 

 speciahzation of limbs and the narrowing of the manner of living 

 to one particular groove as confessions of weakness, the renuncia- 

 tion of the wider Ufe for one that is sharply circumscribed. 



The stock from wliich man eventually emerged played a very- 

 humble r61e for long ages after many other mammalian orders had 

 waxed great and strong. But the race is not always to the swift, 

 and the lowly group of mammals which took advantage of its 

 insignificance to develop its powers evenly and very gradually 

 without sacrificing in narrow specialization any of its possibilities 

 of future achievement, eventually gave birth to the dominant and 

 most intelligent of all living creatures. 



The tree shrews are small squirrel-hke animals which feed on 

 "insects and fruit, which they usually seek in trees, but also occa- 

 sionaUy on the ground, "Wlien feeding, they often sit on their 

 haunches, holding the food, after the manner of squirrels, in their 



