A STUDY OF RECIPROCAL CROSSES. 95 



as to which element, male or female, refuses to fuse with the 

 other. In other words, does the pollen spore fail to take, or does 

 the germ cell fail to receive? From the nature of the case we 

 suppose that the germ cell will not receive ; for the male nucleus 

 has arrived, and therefore, has seemed to have done its work. 

 That the failure to fuse is not due to a difference in structure of 

 the nuclei, is shown by the microscope. The nuclei, so far as the 

 microscope reveals, are of precisely the same size and shape. So 

 that the dissimilarity or differentiation of gametes, if such exist, 

 must be sought in characters which are probably constitutional, 

 perhaps chemical, in nature. 



It has been suggested that the reason why the elements do not 

 unite is because there is a lack of affinity in the plants crossed. 

 But an explanation of this sort does not explain, for we hesitate 

 to define lack of affinity. It is well to refer here to the statements 

 that closeness of relationship marks the limit of successful graft- 

 ing. And the same thing may be said in regard to the limits of 

 crossing. As a rule, only closely related plants can be crossed. 

 Affinity, resemblance, or likeness, allows the union of similar 

 parts. Dissimilarity or unlikeness prohibits union. Now, if we 

 return to reciprocal crosses, we find it difficult to make the appli- 

 cation. How is it that the golden custard squash is so like the 

 crookneck that it will receive its pollen and bear seed ; but the 

 crookneck be so unlike the golden custard as to refuse to be fert- 

 ilized by such pollen? If such conditions exist, they must be^ 

 centered in the sexual elements, the gametes. 



In speaking of the delicate adjustment of plants, Wallace 1 has 

 suggested that the reason certain plants refuse to cross recipro- 

 cally, is an indication of disturbance of the sexual organs. Very 

 likely there is a germ of truth in this suggestion, and we might 

 add that the sexual organs of impossible reciprocals are in a state 

 of unstable equilibrium. This seems rational from the nature of 

 the case ; but what is disturbed ? Is it the molecular swing of the 

 basis of life? Is some constitutional character disturbed? Or 

 is there a disturbance in one or both cases ? This feature of the 

 problem must remain as a matter of conjecture. 



Since there does not appear to be a difference in structure of 

 the gametes, it is at least possible that a disturbance has taken 



1. Alfred R. Wallace, Darwinism, page 155. 



