EEV. G. TTF.yrST.OW — FERTILISATION OF PLANTS. 203 



allowed both kinds to germinate, and as soon as he got pairs of 

 exactly the same height, he planted them on opposite sides of a 

 pot, the mould and moisture, etc., being the same, and the plants 

 subject to exactly the same conditions. Then he allowed these 

 pairs to grow up, and when they were fully groAvn he measured 

 their heights in inches, always calculating the "intercrossed" as 

 100 for convenience, and the resulting ratios were recorded for ten 

 years. He grew five or si:s pots every year, and in eacb pot five 

 or six pairs of plants, thus raising an immense number of plants 

 altogether. He added up the heights, and then divided by the 

 number of plants, so as to get the average. In the fii'st year's 

 growth the heights were as 100 to 76 ; 100 representing the 

 "intercrossed" and 76 representing the self-fertilised. It went 

 down to 68 the third year; and ia every year the self-fertilised 

 fell short of 100. The interpretation of this, therefore, was that 

 intercrossing did a great deal of good, as shown by the " inter- 

 crossed plants " being higher than the " self-fertilised." Grouping 

 the years ia threes, another result comes out. Thus the averages 

 of the first three years give the ratio of 100 to 74. The averages 

 of the fourth, fifth, and sixth years give the ratio of 100 to 78, i.e. 

 they are nearer equality ; and the average of the next three years 

 gives the ratio of 100 to 88, i.e. still nearer equality. Hence the 

 ratio is becoming ajjproximateli/ equal to unity as the generations go on. 

 It shows however that the intercrossing was beneficial for the first 

 few years, but as it proceeded the benefit apparently began to die 

 out, and the plants became approximated to the self-fertilised. 

 You get the very same result when you take the ratios of fertility, 

 as represented by the average number of seeds developed in the 

 capsule. Mr. Darwin has not tabulated this, but I have calculated 

 it from his book. There are two generations in the proportion 

 of 100 to 93 ; the next two generations are as 100 to 94 ; one gener- 

 ation, the fifth, gives a ratio of 100 to 107, while the eighth gives 

 a ratio, by calculation, of 100 to 114! Hence there is a gradual 

 approximation to unity, or 100 to 100, as years go on, in two 

 respects. Intercrossing appears to be beneficial at first ; but after- 

 wards the benefit dies out, and then the plants show no improve- 

 ment upon the self-fertilised. On the contrary, self-fertilisation 

 proves to be more beneficial than intercrossing. 



Tills is also shown in another way. When Mr. Darwin first 

 cultivated the plants, there was an immense variety in the colours ; 

 but subsequently they got less variable, the intercrossed gradually 

 assumed one and the same colour, though never absolute uniformity ; 

 whereas the self-fertiHsed became absolutely uniform in colour. 

 This bi-ings out an important horticultural fact. If a gardener 

 wishes to keep any particular strain, he must be veiy particular 

 not to cross it by another strain, but propagate it either by slips, 

 bulbs, etc., or self -fertilise it, for such fixes the colour. 



We will now consider the crossing of flowers on the same plant. 

 Unfortunately Mr. Darwin has not gone into this so fully, and one 

 cannot therefore draw very safe deductions from his experiments ; 



