SCIENCE. 



535 



greatest extremes of winter temperature, while reptiles 

 are strictly summer animals, those inhabiting the colder 

 zones being forced to hibernate during the winter. 

 Exist ng reptiles, then, have no need of a warmer cov- 

 ering than they possess. Their local ties and lie habits 

 render this sufficient to protect them trom all the changes 

 of temperature to which they are exposed during their 

 period of activity. But if the possession of a hairy cov- 

 ering would have enabled the reptiles of the past to 

 remain active throughout the whole year in cold climates, 

 why was it not developed ? The answer is that it would 

 have been ot no special advantage to them. They are 

 otherwise unfitted for activity during the season of low 

 temperature, and to adapt them to this condition, not 

 only their outer covering needed to be modified, but 

 their internal organization as well. This change in or- 

 ganization has taken place in many cases, ar.d with it 

 the development of a warmer covering than the reptilian 

 coat. But the reptile, thus modified, has lost its reptil- 

 ian character. It has, in the one case, with other less 

 important changes, become a bird ; in the other case, 

 with other more important changes, it has become a 

 mammal. 



The change in internal organization referred to is that 

 in the circulating system. The imperfect heart, the sack- 

 like lung, and the half-aerated blood of the reptile have 

 developed into the perfect heart, the unlike but widely- 

 extended lungs, and the fully aerated blood of the mam- 

 mal and the bird. The varying temperature of the rep- 

 tile is exchanged for the unvarying temperature of his 

 successors. The so-called cold-blooded reptile, with its 

 insufficient oxygenating organs, is at a disadvantage as 

 compared with the bird and the mammal, with their fully 

 oxygenated blood. 



To bodily activity is necessary an internal temperature 

 sufficiently high to render the organic chemistry of the 

 body active. In the lemperature of the tropics, and the 

 summer temperature of the«extra tropical zones, all ani- 

 mals possess this temperature, and none are at a disad- 

 tage in this particular. But the reptile depends diiectly 

 on the solar heat for its temperature, the bird and the 

 mammal do not. Thus when the temperature falls the 

 internal temperature of the reptile similarly decreases, 

 its organic chemical change declines in activity, it be- 

 comes sluggish in movement, unable to obtain food, and 

 would perish but for the hibernating habit which is cus- 

 tomary with it. But the bird and the mammal preserve the 

 temperature essential to organic chemical activity. They 

 conunue, therelore, awake and energetic, and in a con- 

 dition to obtain the necessary food-supply. 



The reptile is essentially a tropical animal. Its or- 

 ganization unfits it for the extremes of extra tropical 

 lemperature, and it is active in the temperate and frigid 

 zones only during the tropic heat of their summers, but 

 conceals itself and continues torpid during the cold of 

 their winters. 



Birds and mammals are essentially adapted to a life in 

 the colder zones. They must have originated in regions in 

 which wintry cold, for some part of the year, replaced the 

 summer heat. The reptilian circulation sufficed for the 

 needs of animals bathed in a fixed degree of external 

 heat, high enough to promote their bodily activity. But 

 animals exposed to severe cold during any portion of the 

 year must either hibernate during that period, or must 

 gain an improved circulation. The heat which fails them 

 wiihcut must be produced within, or their activity must 

 cease. 



This is what we must understand from the systems of 

 circulation of the bird and the mammal. Their reptilian 

 progenitors slo.vly gainsd more complex lungs with an 

 inci eased aerating surface ; the blood became more fully 

 oxygenated ; the arterial and venous blood became more 

 completely separated in the chambers of the heart ; and 

 as a natuial result the internal temperature increased. 

 Such slight changes could not have been preserved and 



augmented unless of advantage. They would have been of 

 no special advantage to the tropical animal. To the animal 

 of the temperate zones they were decidedly advantageous, 

 in enabling it to remain active during a greater portion 

 of the year, and finally during the whole year, the in- 

 ternal stores of heat replacing the lost external stores 

 when winter replaced summer. 



But these internal stores must not only be produced, but 

 must be retained. A heat-retaining covering is necessary 

 to hinder the chilling effect of the wintry air. The reptilian 

 scale is obviously not sufficient for this purpose. As the 

 internal heat of the animal increased, and it was able to 

 prolong its period of active life more and more into the 

 cold season, some modification of the scale became nec- 

 essary, so as to make it more efficient in retaining this 

 internal heat. The scales may. from their points of origin, 

 have grown out longitudinally, covering each other in 

 successive layers, and thus forming a warmer and closer 

 covering. Such a process of elongation, if accompanied 

 by a narrowing of the individual points of origin, would, 

 in time, convert the the scale into a hair. It is well known 

 that they are capable of becoming so converted, by such 

 an elongating outgrowth. 



Thus the haired and feathered animals could not have 

 arisen until the possibly genera) summer of early times 

 was replaced by a double season of summer and winter 

 in the extra tropical regions. But though thus of tem- 

 perate origin, there was nothing to hinder their spreading 

 both into the frigid and the tropic zones. Their improved 

 circulation gave them an activity superior to that of the 

 preceding reptilian rulers of the tropics, and they thus 

 had an advantage in the life battle, which soon showed 

 its effects. The giant reptiles disappeared and giant 

 mammals took their place. Gradually the reptiles re- 

 treated before the march of the mammals. They sank 

 to the ground, hid in holes, learned to creep, to squirm, 

 to swim, while their mammalian successors proudly 

 stalked over their conquered realm, the lords of the eirth. 



If this, through the advantage gained by adaptation to 

 wintry cold, animals were evolved possessed of a perfect 

 circulation, a fixed internal temperature, and a poorly con- 

 ducting external covering of hair ; and if these animals, 

 through their improved powers, banished their reptilian 

 predecessors, or forced them to retreat to the waters, the 

 holes, and the dark recesses of the earth ; it remains to 

 consider the subsequent variations of these hair covered 

 animals ; or, at least, of the flying sections of these crea- 

 tures. 



The next question to be considered is that of the change 

 from a quadrupedal to a bipedal habit of motion. There 

 is only one true biped among the whole great class of mam- 

 mals, namely, man. He is approached in this bipedal 

 habit by the higher apes, and it is not difficult to under- 

 stand how the specialization of limbs took place in the 

 latter. It undoubtedly arose from the climbing habits of 

 monkeys. The fore limbs became used as grasping or- 

 gans, the hind limbs as supporting organs. As climbing 

 monkeys increased in size, they must in many cases have 

 moved by grasping upper branches with their hands, and 

 supporting their feet on lower branches. This was an im- 

 perfect bipedal movement. Eventually some of them be- 

 came too heavy to render a continual arboreal residence 

 desirable. These came to spend the most of their lives 

 upon the earth, as we find in the larger apes of the pres- 

 ent day. But these apes are neither quadrupeds nor 

 bipeds. The specialization of their limbs during a long 

 arboreal residence has unfitted them for either mode of 

 motion upon the ground, and they move along in an awk- 

 ward and inefficient compromise between the two modes 

 of motion. Evidently the method of progression of these 

 animals is not a desirable one for a land residence. Natu- 

 ral selection must tend to make them full quadrupeds or 

 full bipeds. Those of them which have recently changed 

 their arboreal for a ground habitat, have not had time to 

 change. Those which earlier descended to the earth have 



