558 THE GENESIS OF NEW FORMS AS A RESULT OF CROSSING. 



of different constitutions possesses marks and attributes, some of which are charac- 

 teristic of the father and others of the mother. As a matter of fact, the individuals 

 which growers call by the various names of hybrids, mongrels, and bastards answer 

 to this supposition. Some of their attributes and external characters are derived 

 from the paternal, some from the maternal stock. If the relative admixture of the 

 two stocks were to be determined in the case of a hybrid by summing up the 

 characteristics transmitted from each source, the share of each parent would in 

 many cases be found to be a half. In such a case it is usual to say that the hybrid 

 is a mean between its parents. Examples are afforded by Geum hybridum, which 

 originates from crossing Geum montanum and G. rivale, Hieracium stoloniflorum, 

 derived from HieraciuTri aurantiacum and H. pilosellmforme, and Nuphar inter- 

 medium, a cross between Nuphar luteum and N. pumilum. But it also happens 

 sometimes that the proportion of characters inherited by a hybrid from one of the 

 parent-stocks amounts to about two-thirds, leaving only one-third as the propor- 

 tion inherited from the other, and in that case the hybrid is said to exhibit an 

 approximation to one of the parent-species. The Saxifrage hybrids are very 

 instructive examples of this class. The stigmas in a flower of Saxifraga aizoides 

 were dusted with the pollen of S. ccesia. Fertilization was successfully effected, 

 and a capsule containing fertile seeds came to maturity. The plants reared from 

 these seeds resembled in some cases the intermediate form described by Botanists 

 under the name of Saxifraga j^cttens; others approximated more closely to the 

 paternal stock, and a third group to the maternal stock. A similar result was 

 obtained by crossing a flower of Saxifraga aizoides with the pollen of S. mutata. 

 From seeds of the same capsule were produced two different kinds of hybrid, 

 of which one was intermediate between the parents (Saxifraga Hausmanni), 

 whilst the other (Saxifraga inclinata) approximated more closely to S. Tnutata. 

 These experiments point to the conclusion that the share which each parent 

 has in the form of a hybrid depends upon the quantity of its spermatoplasm 

 or ooplasm, as the case may be, used in the production of the seedling relatively 

 to the quantity derived from the other parent; and in the case of hybrids issuing 

 from the same fruit we are forced to suppose that the variability of the degree in 

 which a particular form is inherited is caused by the existence of quantitative 

 differences between the several spermatoplasmic and ooplasmic nuclei (or cells) 

 which coalesce in pairs in the interior of the same ovary. 



Important evidence in support of this supposition is derived from experiments 

 made in connection with the crossing of various composites of the Thistle genus 

 (Cirsium), In these plants each ovary contains a single ovule only, and therefore 

 each fruit can only produce a single individual. On the other hand, each capitulum 

 is composed of a large number of florets, and when a head is in full bloom nearly a 

 hundred mature stigmas project from it in close proximity to one another. If pollen 

 from another species is transferred by means of a paint-brush on to these stigmas, 

 wholesale crossing, so to speak, ensues; and it may be confidently expected that a 

 proportion of these simultaneous crosses will be effectual. The harvest of fruit 



