128 The Outline of Science 



growth of hair, and so forth. In many cases there is an increase 

 of the protection "before the winter sets in, e.g. by growing thicker 

 fur or by accumulating a layer of fat below the skin. 



But the thickening or protection of the skin involved a partial 

 or total loss of the skin as a respiratory surface. There is more 

 oxygen available on dry land than in the water, but it is not so 

 readily captured. Thus we see the importance of moist internal 

 surfaces for capturing the oxygen which has been drawn into the 

 interior of the body into some sort of lung. A unique solution 

 was offered by Tracheate Arthropods, such as Peripatus, 

 Centipedes, Millipedes, and Insects, where the air is carried to 

 every hole and corner of the body by a ramifying system of air- 

 tubes or tracheae. In most animals the blood goes to the air, in 

 insects the air goes to the blood. In the Robber-Crab, which has 

 migrated from the shore inland, the dry air is absorbed by 

 vascular tufts growing under the shelter of the gill-cover. 



The problem of disposing of eggs or young ones is obviously 

 much more difficult on land than in the water. For the water 

 offers an immediate cradle, whereas on the dry land there were 

 many dangers, e.g. of drought, extremes of temperature, and 

 hungry sharp-eyed enemies, which had to be circumvented. So 

 we find all manner of ways in which land animals hide their eggs 

 or their young ones in holes and nests, on herbs and on trees. 

 Some carry their young ones about after they are born, like the 

 Surinam toad and the kangaroo, while others have prolonged the 

 period of antenatal life during which the young ones develop in 

 safety within their mother, and in very intimate partnership with 

 her in the case of the placental mammals. It is very interesting to 

 find that the pioneer animal called Peripatus, which bridges the 

 gap between worms and insects, carries its young for almost a 

 year before birth. 



Enough has been said to show that the successive conquests 

 of the dry land had great evolutionary results. It is hardly too 

 much to say that the invasion which the Amphibians led was the 



