148 EXPERIMENTAL EVOLUTION LECT. 



least in the physiological characters. Of the same 

 brood of pigs the last born is generally weaker ; of 

 twin lambs one is always heavier, etc. And if any two 

 animals of the same brood are selected and observed 

 in after-life in order to estimate the differences 

 between them in physiological respects, although 

 they live under exactly similar conditions, consider- 

 able differences appear in their weight, height, etc. 

 It is enough to note these facts, without dwelling any 

 longer on the topic. But while variability exists 

 everywhere, in every species, in every individual, it 

 must be also admitted that its sum total is very 

 variable. Some species are much more liable to 

 variation than others, as all horticulturists can testify. 

 But, in fact, truth seems to require another statement. 

 All species are probably equally variable, but all are 

 not at the same time in conditions which are equally 

 suitable for the production of variation. Take the 

 case of any imported species of plants which is 

 cultivated in our European gardens. At first no 

 variability is apparent, and many horticulturists give 

 up the hope of improving or of modifying it in any 

 way. But those who have more experience go on 

 with their plant, knowing very well that while at 

 first no marked variability appears, very important 

 variations may suddenly appear at a later stage, after 

 cultivation has been continued for some time. Such 



