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OBSERVATIONS UPON MULING AMONG PLANTS. 



be the case were they mere varieties. The essence of a species 

 consists in the definite relation of its sexual powers to other 

 species, which relation, as well as the specific form, is in every 

 species especial and constant. Form and essence are in this respect 

 one. There are in fact two distinct kinds of affinity, an outer 

 and an inner — one resting on agreement in habit, in stature, in the 

 form of the leaves, in the harmony of the blossoms and organs of 

 fructification ; the other in the greater or less disposition for the 

 sexual union of species in the formation of hybrids ; the one 

 morphological, the other physiological. And inasmuch as both 

 frequently coexist, the more intimate relation is often regarded 

 as identical with, or an immediate consequence of that which is 

 external. And this error is strengthened by the observation of 

 the ease with which closely allied forms, such as those of Melons, 

 Cucumbers, &c, unite. In such cases, however, it is to be re- 

 membered that there is no new formation from two heterogeneous 

 elements, as in the case of real hybrids, but merely the union of 

 the original homogeneous elements of a single species. 



Hitherto the capacity of plants for hybridization has been 

 principally spoken of with reference to the female organs, but 

 the male organs are no less necessary elements. The outer 

 appearance of pollen, its form, size, and colour, seem to have 

 no influence on its fitness for the production of hybrids, sup- 

 posing always that there is no prejudicial operation. As re- 

 gards size, the pollen of Dianthus caryophyllus fructifies D. 

 superbus, the pollen of the latter being much smaller than that 

 of the former. Difference of colour, as in Nicotiana, is unim- 

 portant, except indeed that it has been observed that the union 

 of varieties of Verbascum with similarly coloured pollen is more 

 fruitful. And with respect to the spermatic globules, it is most 

 uncertain whether they perform the same office as those in 

 animals, or whether they are really of the same nature. Dr. 

 Brown's observation that at the time of penetration of the pollen 

 tubes they are few in number, or altogether absent, makes it 

 very doubtful whether they are so analogous as was once sup- 

 posed ; and if not, difference of size, even if always readily appre- 

 ciable, would give no certain indication. 



It appears, therefore, that there is no external or structural 

 character which can determine the aptitude of plants for hybrid 

 union ; the outward agreement in habit is indeed a probable 

 ground of success, but far from infallible, as it depends upon an 

 invisible harmony of inward constitution. The aptitude amongst 

 species even for union is scarcely ever present in an equal 

 degree, nor is it necessarily reciprocal, but greater or lesson one 

 side than the other. 



We have already spoken of the difference as to elective 



