150 Changing Plant and Animal Nature 



The instincts of birds in relation to nest-building, the 

 behavior of the solitary wasp in providing for her offspring 

 which she will never see, the complex relationships among 

 the social insects, and thousands of other cases can be cited 

 to show extremely delicate adaptation of a species to a very 

 specialized mode of life involving at the same time the orien- 

 tation to the physical and material forces, and to many other 

 species of plants and animals, and to other members of the 

 same species, insuring reproduction. 



Among the mammals, which we consider the highest 

 class of vertebrates, the specialization of the class as a whole 

 in relation to reproduction is quite as remarkable as the 

 adaptation found in any single species. Consider for ex- 

 ample the mammary glands and the placenta with the de- 

 velopment of the young inside the mother's body. We see 

 not merely the modification of skin glands to produce a 

 special kind of secretion, the milk, but the fitness of this 

 secretion for the nourishment of the young and its produc- 

 tion at the very time when the offspring is there to use it. 

 We see the native impulses of the young to grasp and suckle 

 at the breast, combined with the readiness of the mother to 

 meet these demands upon her. We see in the placenta an 

 organ made up in part of the tissues of the mother and in 

 part of the tissues of the child. By means of this structure 

 of joint origin, the developing fetus is nourished with ma- 

 terials in the mother's blood, is constantly supplied with 

 oxygen, and is constantly freed of waste material. Yet at 

 no time is there a direct connection between the blood stream 

 of the mother and the blood stream of the fetus. 



In some mammals the details of structure and function 

 that distinguish the species show again a remarkable fitness 

 for special conditions of living, for insuring the production 

 of offspring, and for protecting the young. The early 

 naturalists, like the casual observer today, could not but be 

 impressed by these adaptations. And in proportion as one 

 is impressed, he is likely to consider adaptation the central 

 fact in specific differences. 



