Heredity and Natural Selection. 283 



enough upon earth, to allow all the harmonious adjust- 

 ments of living things to be slowly perfected in this way. 

 The vast number of changes which must be co-ordi- 

 nated in order to produce any considerable modification 

 of one of the higher animals, and the length of time 

 which must be necessary if the successive steps are purely 

 fortuitous, are. points which must have attracted the no- 

 tice of every one who has read the " Origin of Species." 

 The difficulty is obvious, and it has been noticed by 

 many writers, but Murphy, in his discussion of the evo- 

 lution of the vertebrate eye (Habit and Intelligence, 

 p. 319), has stated it with great force. He says: "The 

 higher the organization, whether of an entire organism 

 or of a single organ, the greater is the number of the 

 parts that co-operate, and the more perfect is their co- 

 operation; and consequently the more necessity there is 

 for corresponding variations to take place in all the co- 

 operating parts at once, and the more useless will be any 

 variation Avhatever unless it is accompanied by corre- 

 sponding variations in the co-operating parts; while it is 

 obvious that the greater the number of variations which 

 are needed in order to effect an improvement, the less 

 will be the probability of their all occurring at once. It 

 is no reply to this to say, what no doubt is abstractly 

 true, that whatever is possible becomes probable, if only 

 time enough is allowed. There are improbabilities so- 

 great that the common-sense of mankind treats them as 

 impossibilities. It is not, for instance, in the strictest 

 sense of the word, impossible that a poem and a math- 

 ematical proposition should be obtained by the process 

 of shaking letters out of a box; but it is improbable to 

 a degree that cannot be distinguished from impossibil- 

 ity; and the improbability of obtaining an improvement 

 in an organ by means of several spontaneous variations, 

 all occurring together, is an improbability of the same 



