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IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
each other. This latter statement is clearly proved by the differ- 
ence in the result of reciprocal crosses between the same two 
species, for according as the one species or the other is used as 
the father or the mother, there is generally some difference, and 
occasionally the widest possible difference in the facility of effect- 
ing a union. The hybrids, moreover, produced from reciprocal 
crosses often differ in fertility.” 
Again he says: “There is often the widest possible difference 
in the facility of making reciprocal crosses. Such cases are 
highly important, for they prove that the capacity in any two 
species to cross is often completely independent of their systematic 
affinity, that is, of any difference in their structure or constitution, 
excepting in their reproductive systems. It can thus be shown 
that neither sterility nor fertility affords any certain distinction 
between species and varieties. The evidence from this source 
graduates away, and is doubtful in the same degree as is the 
evidence derived from other constitutional and structural dif- 
ferences.” 
Darwin finally summarizes the evidences as follows: “First 
crosses between forms, sufficiently distinct to be marked as species, 
and their hybrids, are very generally, but not universally sterile. 
The sterility is of all degrees and is often so slight that the most 
careful experimentalists have arrived at diametrically opposite 
conclusions in ranking forms by this test.” 
In 1861 the Paris Academy of Sciences proposed the following 
problem to receive the grand prize in the physical sciences : “To 
study the plant hybrids from the point of view of their fecundity, 
and of the perpetuity or non-perpetuity of their characters. The 
production of hybrids among plants of different species of the 
same genus is a fact determined a long time since, but many pre- 
cise researches still remain to be made in order to solve the fol- 
lowing questions, which have an interest equally from the point 
of view of general physiology, and of the determination of the 
limits of species, of the extent of their variations. 
“1. In what cases of hybrids are they self-fertile? Does this 
fecundity of hybrids stand in relation to the external resemblances 
of the species from which they come, or does it testify to a 
special affinity from the point of view of fertilization, as has been 
remarked regarding the ease of production of the hybrids them- 
selves?” 
“2. Do self-sterile hybrids always owe their stability to the 
imperfection of the pollen? Are the pistils and the ovules always 
