174 LECTUKES ON BIOLOGY 



for fitness must take place, i.e., that of the adult animal, becomes 

 in consequence very limited. In addition to the immense num- 

 ber, the accidental play of natural selection requires equally 

 immense periods of time. The question arises therefore whether 

 the geological periods are actually long enough to make the 

 genesis of all the useful characteristics within that time conceiv- 

 able. It is true that at the first glance the geological period 

 appears enormously great. Some geologists estimate that since 

 the Cambrian period up to modern times 200 millions of years 

 have elapsed ; yet what an enormous labour is Nature expected to 

 have performed since then ! Mayer-Eymar estimates the Tertiary 

 period to have lasted 325,000 years, while others estimate it at 

 2,350,000 years. But when we consider that within this time all 

 the higher mammals and all their numerous adaptations are said 

 to have been originated, even this immense period seems much 

 too short for a pure play of accident. 



While natural selection excels artificial selection in that it 

 is able to commence in many points at the same time, it is 

 decidedly inferior to it in one important respect. If in an 

 enormous number of domesticated animals there appears only in 

 two or three individuals a variation which seems useful to man, 

 he is able to select those animals among thousands, cross them, 

 and thus maintain and increase that particular variation. But 

 let us suppose that something similar happens in Nature. Let us 

 assume that of the hares in a certain district two are by the 

 colour of their fur better adapted to their surroundings than the 

 rest. Is it safe to assume, or is it even probable, that because of 

 this little advantage these two, and only these two, animals escape 

 the persecution of their enemies ? Even supposing a lucky acci- 

 dent so to ordain it that they do remain alive, would it not be in 

 the nature of an extraordinary miracle if these two animals with 

 the useful variations found each other and paired with each 

 other ? Must we not rather assume that the pairing would take 

 place with one of the other hares, and that thus the distinctive 

 feature would after a few generations disappear again? Assuredly 

 we cannot assume that all the hares not thus distinguished would 

 be suddenly exterminated. For if the hares were previously able 

 to maintain their species without this protective character, it is 

 unintelligible why it should be difficult now, so long as conditions 



