206 SCIENCE PROGRESS. 



paribus, be inferior to the offspring of cross-fertilisation, if 

 placed in direct competition with the?n. 



At the same time that these researches were going on, 

 many cases of regular self-fertilisation were being discovered, 

 though in all of them, as the flowers were open and often 

 contained honey, there was a possibility of a cross taking 

 place. Miiller pointed this out, and proposed to substitute 

 for the Knight-Darwin hypothesis, another, based upon the 

 above facts, viz., "that cross-fertilisation results in offspring 

 which vanquish the offspring of self-fertilisation in the 

 struggle for existence ". Cross-fertilisation was still re- 

 garded as the chief object, so to speak, of a flower, self- 

 fertilisation as a pis aller. 



Until recently no cases were known of plants which 

 always fertilise themselves and had no chance of a cross. 

 Burck has shown that Myrmecodia and other plants never 

 bear open flowers, but always fertilise themselves in cleisto- 

 gamic flowers. 1 Hence the Knight-Darwin hypothesis must 

 be abandoned in favour of some more general hypothesis, 

 which will include these cases. Naegeli, as a result of his 

 experiments upon hybridisation, laid down the law that 

 " the consequences of fertilisation reach their optimum when 

 a certain mean difference in the origin of the sexual cells is 

 attained ". The experiments of Darwin and others support 

 this law, though they are contradictory when applied to the 

 Knight- Darwin hypothesis. The latter is seen to be a 

 corollary, for certain cases, of Naegeli's law. We may, 

 therefore, for the present, adopt this law as a basis for 

 further study ; MacLeod proposes further hypotheses wider 

 than that of Miiller, based upon this law, and upon Darwin's 

 experiments. 



That cross-fertilisation, in itself, is advantageous, is 

 probable for most cases, but there are at least three factors 

 in the resulting gain, which ought to be considered separately 

 and not lumped together as is usually done. These are (a) 

 fertility of the parent plant, (6) strength of constitution of 

 the offspring, (c) fertility of the offspring. Darwin measured 



1 Cleistogamic flowers = closed self-fertilising flowers. 



