978 LIFE : OUTLINES OF GENERAL BIOLOGY 



variation in a particular direction may become cumulative, and 

 may be progressive in the sense that it increases from generation 

 to generation. This might be accounted for by the transmission of 

 acquired characters, but the possibility of this has not been con- 

 vincingly demonstrated. It might also be due to the pairing of 

 individuals possessing the same characteristic in a marked degree. 

 Yet it may perhaps be due simply to what we must call, for lack 

 of a better term, evolutionary momentum. And in this, as elsewhere, 

 "we may have too much of a good thing". Thus progressive varia- 

 tion in a surely somewhat unprofitable direction has led to the 

 male narwhal's enormous six-foot-long incisor tooth, or to the 

 over- weigh ting of the Giant Irish Deer with enormous antlers; or 

 to the over-thickening of the bony ramparts of the extinct Ground 

 Sloth. Apart from cases where there is good reason to postulate a 

 long-continued process of stringent selection, there is considerable 

 evidence of an organic momentum increasing from generation to 

 generation, and sometimes even to becoming fatal. Moreover, when 

 a structure, slowly evolving for ages, attained to a new utility, 

 there would also be an internal pulse. Thus there must have been 

 an evolutionary pulse in the life of every type whose eye passed 

 from being a light-and-shade organ to be image-forming, or whose 

 ear passed from being a balancing organ to function as a hearing 

 ear. The progressive variation which led to the establishment of the 

 neo-pallium in the fore-brain of certain mammahan types must have 

 meant an evolutionary pulse, and the same must be said of every 

 advance that gave its possessors in some appreciable degree a new 

 world. We must thus later consider more carefully what such 

 evolutionary pulses may be. 



WHAT ARE GENES? 



What Galton called "the natural inheritance", inconceivably 

 condensed, is carried by the egg-ceUs and the sperm-cells, and 

 though it would be rash to conclude that the cell-substance does not 

 carry some of the old-established items in the inheritance, the evi- 

 dence is strong that the nuclei of the germ-ceUs are the chief vehicles 

 of the hereditary characters which persist from generation to 

 generation. But in certain conditions the complex, readily stainable, 

 chromatin material of the nucleus of the germ-cell takes the form 

 of well-defined chromosomes; and it is a widely accepted view that 

 the chromosomes are the vehicles of the hereditary characters. 

 Fixed and stained, the chromosomes often look so extraordinarily 

 weU-defined that we are apt to forget that in their living state they 

 seem to be differentiations of fluid, surrounded by a film. Yet we 

 know that each species has its own particular number of chromo- 



