EVOLUTION 1045 



some measure demonstrable, because it illustrates an adjustment 

 that can only be established by intimate inquiry. It is highly 

 probable that similar adjustments are exhibited by many animals 

 when they have to face a marked climatic change. 



Many facts confirm the suggestion that the success or failure of 

 attempted naturalisation may depend on inconspicuous constitu- 

 tional peculiarities. Thus G. M. Thomson (1922) notes for New 

 Zealand that the greenfinch and the chaffinch have thriven remark- 

 ably, while the allied linnet has quite failed. "The reasons for these 

 failures are often so obscure that no plausible explanation has yet 

 been given." "The Pacific-coast salmon (Onchorhynchtis qidnnat) 

 has become strongly established on the east coast of the South 

 Island; while all attempts to naturalise the Atlantic Salmon (Salmo 

 salar), though carried on unceasingly for half a century and in half 

 a hundred different streams, have absolutely failed." Further 

 biological inquiry should be made into the reasons for failure. 



A hint of the frequent subtlety of conditions may be found in 

 cases where the attempted naturalisation of a plant fails, as of 

 heather in Ceylon, because the associated root-fungus or mycorhiza 

 will not grow. Many orchids in Europe have had the like difficulty, 

 until the mycorhiza the seedlings needed was supplied. According 

 to some botanists, however, the obligatoriness of the symbiosis has 

 been exaggerated. 



(7) When an organismal change directly induced by some change 

 in environment, nutrition, or habit, takes such a grip that it per- 

 sists after the inducing conditions have ceased to operate, it is 

 called a modification, or, less conveniently, an individually acquired 

 character. There seem to be some climatic modifications, and the 

 following may be mentioned, {a) An Englishman who works half 

 his lifetime under a tropical sun may become so tanned that the 

 deposit of melanin pigment in the skin does not disappear during 

 all the years in which he enjoys his pension at home. He has changed 

 his skin, but he cannot change it back again. Of course it must not 

 be inferred that the blackness of the negro's skin was directly pro- 

 duced in this way by the tropical sun. (h) Nageli brought some 

 Alpine plants to the Botanical Garden at Munich, and there many 

 of them became in the first year so much modified that they were 

 hardly recognisable as the same species. Their descendants in the 

 garden were also quite different from the Alpine originals. Thus the 

 small hawkweeds (Hieracium) became large and thickly branching 

 with abundant blossoms. The modifications were very striking, and 

 in some cases many generations were observed — even for thirteen 

 years. The persistence was probably due to the fact that the 

 original modifications were directly re-impressed on each successive 

 crop, for when the plants were removed from the rich garden to 

 poor, gravelly soil, the acquired characters gradually disappeared. 



