i8 3 THE POPULAR SCIENCE MONTHLY. 



undeviating arrangement of leaves, to be the distributing of leaves 

 most rapidly" and thoroughly around the stem, exposed more com- 

 pletely to light and air, and provided with greater freedom for sym- 

 metrical expansion, together with more compact arrangement of 

 bud; " and he asks, " What has determined such an arrangement of 

 vital forces ? " Theory of types would say, their very nature, or an 

 ultimate creative power. Theory of adaptation would say, the ne- 

 cessity of their lives, both outward and inward ; or the conditions, 

 both past and present, of their existence. 



Whatever tends to show modification in the markings, color, size, 

 food, or change in the variety of habits manifested by animals, fur- 

 nishes just so many indications of the unstable character of what had 

 before been considered stable, and gives an infinitely wider field for 

 those unconscious selections whose operations are coincident with 

 every change in the physical features of the earth. On the theory 

 of derivation additional confirmation is given to the deductions of 

 geologists based upon the stratigraphical and paleontological evi- 

 dences of the rocks. The survival of a marine crustacean in the deep- 

 er waters of Lake Michigan, as discovered by Stimpson, coupled with 

 similar occurrences in the lakes of Sweden, suggests the past connec- 

 tion of these waters with the ocean. In the same way the persistence 

 of arctic forms on high mountain-tops indicates the existence in past 

 times of wide-spread glacial fields. The interesting discoveries of Mr. 

 Ernest Ingersoll, in the Rocky Mountains, of the occurrence of two 

 species of marine mollusks and living crabs belonging to marine forms, 

 a*nd tiny air-breathing mollusks peculiar to the Gulf coast and West 

 Indies, point as distinctly to the past connection of that region with 

 the ocean as the records of marine life left in the rocks. And more 

 than this, the survival of these few forms gives us a conception of the 

 thousands of animals which have succumbed to the changed condi- 

 tions. Connected with the evidences of recent elevation of this re- 

 gion are the discoveries of Marsh in finding that, when the gill-bear- 

 ing salamander Siredon is brought down from the colder waters of 

 the Rocky Mountains to the warmer waters below, a complete change 

 takes place in a loss of the gills and the conversion of the animal 

 into the air-breathing genus Amblystoma. 



This exhibits on a wider scale the experiments often performed in 

 keeping tadpoles in the dark and cold, and indefinitely retarding 

 their development, thus forcing them, as it were, to retain their ear- 

 lier condition. Among the many millions of individuals of Ambly- 

 stoma, some must have presented the anomaly of a premature devel- 

 opment of their ovaries before the larval stage had passed away 

 (similar cases being observed among insects), and thus it has been 

 possible for them to perpetuate their kind in this stage. The Axo- 

 lotl, having the longest persisted in this mode of growth, has be- 

 come, as it were, almost fixed in these retrograde characters, only a 



