§ IV.] TAe question of species. 31 



forms which make their appearance one after another at the 

 same spot, or by the construction of a hypothetical series of de- 

 velopments by the most exact and ingenious comparison of 

 these forms, involves a logical fallacy. We distinguish, for in- 

 stance, a species of wheat by its seed, its stem and leaves, its 

 flowers and fruits, and we know that these proceed alternately 

 from one another ; but the latter fact we know only by ob- 

 serving that the one of these members arises as part of one of 

 the others, and by observing also how this happens. This is 

 the only reason why we consider the grain of wheat to belong 

 to the wheat-plant, whether it is attached to it or has fallen to 

 the ground or lies thrashed out on the floor of the granary. 

 That the stem with the leaves belongs to the grain we know by 

 observing its origin as part of the grain, not because we have 

 seen wheat-plants growing where wheat was sown ; weeds may 

 grow at the same spot along with the wheat. 



This mode of viewing the matter sounds trivial ; its truth will 

 seem obvious to every one, and rightly so ; and yet it cannot be 

 too often repeated, for the logic which it is intended to illustrate 

 is being constantly disregarded, and a mass of confusion has 

 been the result of this neglect. This may be shown by means 

 of the very example which we have chosen, for less than fifty 

 years ago it was maintained that all sorts of weeds were pro- 

 duced from the seed of the wheat-plant, and people (8) in other 

 respects well-educated and intelligent believed that this was 

 possible, because these weeds sprang up in the spots where 

 wheat had been sown. But whoever examines at the right place 

 finds that either wheat or nothing grows from the wheat-grain, 

 that the weed springs only from the seed of the species of 

 weed which may happen to be present, and that where 

 the weed grows up instead of or with the wheat, its seed has 

 found its way by some means to the place where the wheat 

 was sown. 



Notions and mistakes like these in the case of the wheat- 

 plant have appeared again and again in connection with smaller 



