2o8 SCIENCE PROGRESS. 



make up for every plant in any locality a sort of balance- 

 sheet of profit and loss before we can fully understand its 

 floral mechanism. Only when the gain derived from cross- 

 fertilisation is greater than the price to be paid for it, will 

 it prove advantageous and be retained by the plant. It is 

 important to remember, in this connection, that in all 

 probability the flowering plants began with cross-fertilisa- 

 tion, handed down to them by their cryptogamic ancestors. 



Upon this enlarged basis we can now understand better 

 the great variation in floral mechanism exhibited by the 

 same species in different parts of its distribution area, and 

 it becomes of interest to study it in every possible locality. 

 We might suppose such a case as this : Two plants A and 

 B of different species are removed from a district rich in 

 insects, where they were both crossed, to a district poor in 

 insects. If crossing had proved of great advantage to A, 

 but of small to B, then we may expect to find that A will 

 acquire increased attractiveness, paying the extra price de- 

 manded for cross-fertilisation, whilst B will tend towards 

 self-fertilisation, the extra price being too great to make 

 crossing any longer an advantage, whilst by reduction of 

 corolla, etc., a distinct saving is made, which may be de- 

 voted to extra storage in the seeds, vegetative reproduction, 

 or other purposes. It will be asked, why do not such 

 flowers as B become cleistogamic, and so make the utmost 

 possible saving ? On the older view, it was said that they 

 remained open upon the chance of obtaining a cross, but this 

 no longer suffices as an explanation. It seems possible that 

 the opening of the flower, the honey-secretion, etc., may be 

 useless survivals from insect-fertilised ancestors, and that 

 the necessary circumstances (see below) have not arisen 

 which would cause their complete reduction. 



The world-wide distribution of many self-fertilised plants 

 has always been difficult to explain, but Wallace has pro- 

 posed a solution which harmonises with Kaegeli's law, 

 suggesting that small changes in conditions of life, such as 

 are obtained by moving into new localities, may yield the 

 same advantages as crossing- of different stocks in increased 

 fertility and flexibility of constitution. The wide distribu- 



