tHE RABBIT. 127 



thirteen (chap, vii., 4); and that some have a trans- 

 verse anastomosis between the right and left external 

 jugular veins, while others have not (chap, iii., 8). If 

 we were to undertake the study of a series of rabbits in 

 more minute detail, we should soon be able to add numerous 

 other points of 'difference to the list. 



The fact that the individuals comprised in a species differ 

 from one another is expressed by saying that they show 

 variation. It implies an imperfection of heredity, or 

 tendency of offspring to differ from their parents (vari- 

 ability). It is worth while pointing out, however, that 

 although a new kind of variationfis in the first place due 

 to imperfect heredity or variability, yet this variation 

 becomes itself subject to heredity in the next generation. 

 Hence heredity and variability are both concerned in the 

 existence of variation ; and the student should distinguish 

 between variation and variability. 



3. Adaptation to Conditions of Life, Every differ- 

 ence in details of structure means a corresponding difference 

 in the details of the various functions and activities in which 

 those structures are concerned. A thirteen-ribbed rabbit 

 does not breathe exactly in the same way as a twelve-ribbed 

 one ; and the flow of blood from the head and the variations 

 in blood -pressure are not the same under similar circum- 

 stances in two rabbits, one of which has a jugular anasto- 

 mosis while the other has not. When the variation in 

 structure from the average is very great indeed as when 

 an animal is born with two heads, or without legs the 

 difference in functions and activities is so great that the 

 animal is incapable of living except under very careful 

 artificial treatment. We say that such an animal is a 

 " monster " or " abnormal " or shows " malformation," 

 because its variation is beyond the limits compatible with 

 living an ordinary rabbit's life. Such extreme variations 

 are rare, because they do not tend to be perpetuated by 

 heredity, their possessors usually not living long enough to 

 become parents. The variations commonly met with in 

 any species are within the range of adaptation to the 

 conditions of life of that species, because, if the structure 



