THE GENESIS OP NEW SPECIES. 593 



species in the past will be discussed. They here bring to a natural conclusion a 

 series of examples adduced to show in what manner a genesis of new species may 

 ensue in the present, and may have taken place in the past. No sharp line of 

 demarcation is to be found between different epochs in this connection any more 

 than in the case of any of the other phenomena which, in the aggregate, constitute 

 the history of species. 



Now that it has been shown how new species arise from hybrids, or, in other 

 words, from the crossing of species in pairs, the question presents itself whether, in 

 addition to this one method, there are not also others leading to the same result. 

 In answering this question we must bear in mind that every permanent change in 

 external form which is inherited by a plant's descendants must be preceded by a 

 change in the constitution of the protoplasm, and that so far as investigation has 

 elicited the facts, the centre of the change is located exclusively in a particular 

 protoplast which lies hidden in the ovary and there receives the spermatoplasm. 

 The stimulus which causes the change in this protoplast can only proceed from the 

 spermatoplasm, and every speculation concerning the formation of new species must 

 therefore be associated with the question whether in the intercrossing of plants of 

 one species and in autogamy the protoplasm in the course of its journey to the 

 ooplasm may, as a result of its exposure to new external conditions, undergo modi- 

 fications of so fundamental a kind that its influence on the ooplasm is subject to 

 corresponding variations. In the first place, it might be imagined that the pollinated 

 stigmas do not always act in the same way upon the spermatoplasm of the pollen- 

 cell. Reference has already been made to the fact that a stigma may sometimes be 

 almost simultaneously dusted with the pollen of very different plants (see p. 404), 

 but that it has the power of exercising a selection, and that in every case only one 

 kind of pollen is induced to put forth tubes by which a real fertilization is accom- 

 plished. The other kinds of pollen upon the stigma are not known to have a direct 

 eflect upon the ovule. But that there is some interaction between them and the 

 protoplasm in the cells of the stigma is evidenced by the fact that they swell up 

 wherever they are in contact, and (as has been shown, p. 414) are frequently found 

 developing pollen-tubes. Now it is possible that the reciprocal action of the contents 

 of these pollen-cells and the contents of the stigmatic cells may produce some change 

 in the latter, which is transmitted to the contents of those other pollen-tubes which 

 are to enter into combination with the ooplasm. Such modification might conceiv- 

 ably affect the nature of the stimulus imparted to the ooplasm, and this alteration 

 in the stimulus might be manifested in a change in the form of the individual 

 arising from the fertilized ooplasm. The likelihood of all these possibilities and 

 assumptions being satisfied is extremely small, but as no researches have yet been 

 instituted into the matter, it cannot be dismissed with an unconditional negative. 



In artificial crosses between different species of Cirsium it has often been noticed 

 that pollen-cells taken from a single capitulum vary in their effects upon the stigmas 

 of a second capitulum, inasmuch as the seeds produced by the difierent florets, 

 though all fertilized with the same kind of pollen, yield dissimilar plants when 



Vol. II. 88 



