284 HIGH SCHOOL ZOOLOGY. 



therefore, rendered better fitted to survive, and will not only 

 do so, but will transmit their peculiar variations to their off- 

 spring ; selection being thus made by Nature of the individuals 

 best fitted to the existing conditions of life. 



20. If those conditions were staV)le, a persistent equilibrium 

 of organic nature would be established, and no opportunities 

 for the development of new species would arise, but the condi- 

 tions of life are never stable, and consequently we have a greater 

 or less degree of instability of the balance of life, depending on the 

 greater or less changes taking place in the conditions of exist- 

 ence. It is obvious that any apparent equilibrium would be 

 up.set by changes, however gradual, of the character indicated in 

 § 14 and 15, and, therefore, that such changes must be regarded 

 as those most operative in the pi'oduction of new forms. A species 

 subjected to such changes must either produce variations fitted to 

 cope with them, or else become extinct, perishing at once, or 

 struggling on under the unfavourable influences, till a depau- 

 perate condition, such as has been observed in many fossils, as 

 well as in living forms, is gradually reached. If it is a form 

 capable of adapting itself, by assuming new habits, it is possible 

 that (as we see in varieties spread over a Avide area) this accom- 

 modation should proceed along different lines with different 

 varieties, and therefore, that several new forms would result from 

 the original species. It is evident that the intermediate forms be- 

 ins liker the original in character would tend to be eliminated 

 by their relative unsuital)ility to the new conditions, and thus, 

 instead of a series of vai'ieties, we should have two or more new 

 ' species ' distinct from each other. Another circumstance, 

 tending in this direction, is that individuals of a variety tend 

 to keep together, with the result that comparatively near species 

 become incapable of crossing, so as to form a mixed race. This 

 was at one time considered to be an essential feature in the 

 definition of a species, but several examples are now known 

 of undoubtedly distinct species, which do form such crosses, 

 ■whei-e their ai-eas of distribution come into contact. 



