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。 The first were obviously superior to the latter。
Darwin was surprised by this observation； as he had expected a prejudicial influence of self…fertilisation to manifest itself after a series of generations： 〃I always supposed until lately that no evil effects would be visible until after several generations of self…fertilisation； but now I see that one generation sometimes suffices and the existence of dimorphic plants and all the wonderful contrivances of orchids are quite intelligible to me。〃 （〃More Letters〃； Vol。 II。 page 373。）
The observations on Linaria and the investigations of the results of legitimate and illegitimate fertilisation in heterostyled plants were apparently the beginning of a long series of experiments。 These were concerned with plants of different families and led to results which are of fundamental importance for a true explanation of sexual reproduction。
The experiments were so arranged that plants were shielded from insect… visits by a net。 Some flowers were then pollinated with their own pollen； others with pollen from another plant of the same species。 The seeds were germinated on moist sand； two seedlings of the same age； one from a cross and the other from a self…fertilised flower； were selected and planted on opposite sides of the same pot。 They grew therefore under identical external conditions； it was thus possible to compare their peculiarities such as height； weight； fruiting capacity； etc。 In other cases the seedlings were placed near to one another in the open and in this way their capacity of resisting unfavourable external conditions was tested。 The experiments were in some cases continued to the tenth generation and the flowers were crossed in different ways。 We see； therefore； that this book also represents an enormous amount of most careful and patient original work。
The general result obtained is that plants produced as the result of cross… fertilisation are superior； in the majority of cases； to those produced as the result of self…fertilisation； in height； resistance to external injurious influences； and in seed…production。
Ipomoea purpurea may be quoted as an example。 If we express the result of cross…fertilisation by 100； we obtain the following numbers for the fertilised plants。
Generation。 Height。 Number of seeds。
1 100 ： 76 100 ： 64 2 100 ： 79 … 3 100 ： 68 100 ： 94 4 100 ： 86 100 ： 94 5 100 ： 75 100 ： 89 6 100 ： 72 … 7 100 ： 81 … 8 100 ： 85 … 9 100 ： 79 100 ： 26 （Number of capsules） 10 100 ： 54 …
Taking the average； the ratio as regards growth is 100：77。 The considerable superiority of the crossed plants is apparent in the first generation and is not increased in the following generations； but there is some fluctuation about the average ratio。 The numbers representing the fertility of crossed and self…fertilised plants are more difficult to compare with accuracy； the superiority of the crossed plants is chiefly explained by the fact that they produce a much larger number of capsules； not because there are on the average more seeds in each capsule。 The ratio of the capsules was； e。g。 in the third generation； 100：38； that of the seeds in the capsules 100：94。 It is also especially noteworthy that in the self…fertilised plants the anthers were smaller and contained a smaller amount of pollen； and in the eighth generation the reduced fertility showed itself in a form which is often found in hybrids； that is the first flowers were sterile。 （Complete sterility was not found in any of the plants investigated by Darwin。 Others appear to be more sensitive； Cluer found Zea Mais 〃almost sterile〃 after three generations of self…fertilisation。 （Cf。 Fruwirth； 〃Die Zuchtung der Landwirtschaftlichen Kulturpflanzen〃； Berlin； 1904； II。 page 6。）
The superiority of crossed individuals is not exhibited in the same way in all plants。 For example in Eschscholzia californica the crossed seedlings do not exceed the self…fertilised in height and vigour； but the crossing considerably increases the plant's capacity for flower…production； and the seedlings from such a mother…plant are more fertile。
The conception implied by the term crossing requires a closer analysis。 As in the majority of plants； a large number of flowers are in bloom at the same time on one and the same plant； it follows that insects visiting the flowers often carry pollen from one flower to another of the same stock。 Has this method； which is spoken of as Geitonogamy； the same influence as crossing with pollen from another plant？ The results of Darwin's experiments with different plants （Ipomoea purpurea； Digitalis purpurea； Mimulus luteus； Pelargonium； Origanum） were not in complete agreement； but on the whole they pointed to the conclusion that Geitonogamy shows no superiority over self…fertilisation （Autogamy）。 （Similarly crossing in the case of flowers of Pelargonium zonale； which belong to plants raised from cuttings from the same parent； shows no superiority over self… fertilisation。） Darwin； however； considered it possible that this may sometimes be the case。 〃The sexual elements in the flowers on the same plant can rarely have been differentiated； though this is possible； as flower…buds are in one sense distinct individuals； sometimes varying and differing from one another in structure or constitution。〃 （〃Cross and Self fertilisation〃 （1st edition）； page 444。）
As regards the importance of this question from the point of view of the significance of cross…fertilisation in general； it may be noted that later observers have definitely discovered a difference between the results of autogamy and geitonogamy。 Gilley and Fruwirth found that in Brassica Napus； the length and weight of the fruits as also the total weight of the seeds in a single fruit were less in the case of autogamy than in geitonogamy。 With Sinapis alba a better crop of seeds was obtained after geitonogamy； and in the Sugar Beet the average weight of a fruit in the case of a self…fertilised plant was 0。009 gr。； from geitonogamy 0。012 gr。； and on cross…fertilisation 0。013 gr。
On the whole； however； the results of geitonogamy show that the favourable effects of cross…fertilisation do not depend simply on the fact that the pollen of one flower is conveyed to the stigma of another。 But the plants which are crossed must in some way be different。 If plants of Ipomoea purpurea （and Mimulus luteus） which have been self…fertilised for seven generations and grown under the same conditions of cultivation are crossed together； the plants so crossed would not be superior to the self… fertilised； on the other hand crossing with a fresh stock at once proves very advantageous。 The favourable effect of crossing is only apparent； therefore； if the parent plants are grown under different conditions or if they belong to different varieties。 〃It is really wonderful what an effect pollen from a distinct seedling plant； which has been exposed to different conditions of life； has on the offspring in comparison with pollen from the same flower or from a distinct individual； but which has been long subjected to the same conditions。 The subject bears on the very principle of life； which seems almost to require changes in the conditions。〃 （〃More Letters〃； Vol。 II。 page 406。）
The fertilitymeasured by the number or weight of the seeds produced by an equal number of plantsnoticed under different conditions of fertilisation may be quoted in illustration。
On crossing On crossing On self… with a fresh plants of the fertilisation stock same stock Mimuleus luteus （First and ninth generation） 100 4 3
Eschscholzia californica （second generation） 100 45 40
Dianthus caryophyllus （third and fourth generation） 100 45 33
Petunia violacea 100 54 46
Crossing under very similar conditions shows； therefore； that the difference between the sexual cells is smaller and thus the result of crossing is only slightly superior to that given by self…fertilisation。 Is； then； the favourable result of crossing with a foreign stock to be attributed to the fact that this belongs to another systematic entity or to the fact that the plants； though belonging to the same entity were exposed to different conditions？ This is a point on which further researches must be taken into account； especially since the analysis of the systematic entities has been much more thorough than formerly。 （In the case of garden plants； as Darwin to a large extent claimed； it is not easy to say whether two individuals really belong to the same variety； as they are usually of hybrid origin。 In some instances （Petunia； Iberis） the fresh stock employed by Darwin possessed flowers differing in colour from those of the plant crossed with it。） We know that most of Linneaus's species are compound species； frequently consisting of a very large number of smaller or elementary species former