306 THE LIFE OF THE PLANT 



be proved with regard to organisms in their natural 

 conditions, since we cannot keep them under observation 

 for a sufficiently long period of time. The solution of 

 this problem, though still presenting great difficulties, 

 is becoming a little simpler in I'elation to cultivated 

 plants and domesticated animals, concerning which his- 

 torical data do exist. Our greatest difficulty is to prove 

 that different races of animals or plants, sharply differen- 

 tiated one from the other, do actually descend from 

 one and the same species. Darwin managed to prove 

 it fully in some cases, particularly with regard to the 

 breeds of pigeons. He proved that contemporary breeds 

 vary so much that had they been discovered in their 

 natural environment they would not only have been 

 classified as different .species but even in different genera, 

 and yet they did undoubtedly arise from one and the 

 same pigeon. 



Those who maintain the hypothesis of the immuta- 

 bility of species have one more argument in reserve, and 

 also a very important one. They say : Granted that 

 species descend one from another, how can we account 

 for the absence of all the minute transitional forms which 

 must have existed ? Why have they disappeared ? Why 

 is it that species generally represent groups of beings 

 entirely distinct from each other ? In passing let us 

 remember that the very fact of the absence of transi- 

 tional forms between species is in many cases doubtful, 

 because, as we have already seen, whenever such 

 transition is manifested between two species, the 

 exponents of the immutability of species deny them 

 specific rank. However, this argument holds good in 

 the great majority of cases : really good species, as a 

 matter of fact, are not connected with each other by 

 transitional forms, and this fact was the main obstacle 

 to all the early endeavours to prove the origin of 

 species by means of variation ; they all collapsed in 

 face of this crushing argument. As we proceed to study 



