2j8 INFLUENCE OF CORRELATION AND EXTERNAL STIMULI 



It is however possible to assume that the organisms were from the beginning 

 dependent in the formation of their organs upon external conditions, and 

 that only those of the relationships of dependence which were of use 

 have been retained through the survival of the fittest. This appears to 

 me specially to be supported by the circumstance that often the factor 

 which causes a definite relationship in the formation of organs is not by 

 any means that to which the organ itself is ' adapted.' I have already 

 referred to such examples. The archegonia of the prothalli of ferns 

 arise upon the shaded side. This is of itself a fact of indifference for 

 the function of the archegonia, because in many other cases they arise on 

 the illuminated side of a dorsiventral thallus, as in Pellia and others, or 

 they are distributed equally all round, as in prothalli of Lycopodium, 

 the archegoniophore of the prothalli of Trichomanes, and elsewhere. 

 In the prothalli of ferns however the position upon the shaded side is of 

 advantage because it finds there most easily drops of water which are 

 necessary for the opening and for the fertilization of the archegonia. 



A further peculiarity which must be noted here is that one and 

 the same relationship of configuration may be brought about by different 

 external influences. The motile developmental stages of Myxomycetes 

 swarm-cells, amoebae, plasmodia for example, have the power to pass 

 over into the resting condition, a stage in which they arc very different 

 in configuration from what they were before. The arrest which this change 

 causes can be brought about also by slow drying as well as by other 

 external influences, such as unfavourable nutrition. In correspondence 

 with this I may also mention the aquatic spermaphyte Myriophyllum l , 

 whose peculiar resting condition, which usually develops in the form of 

 a winter-bud at the end of the vegetative period, just like the sclerotium 

 of a fungus, may be called forth at any time by starvation. These plants 

 react then evidently to any unfavourable external condition in this way 

 they pass into resting stages, supposing always that the necessary plastic 

 material is in existence for such formations. Bacteria also behave in 

 the same way in formation of their spores. 



The appearance of ' reversion-shoots,' which exhibit the simple 

 relationships of configuration shown first of all in germination, is brought 

 about in many plants by different conditions which exercise an unfavour- 

 able influence on vegetation ; in this way arise the juvenile forms of 

 leaves in Sagittaria natans, Veronica cupressoides, and others 2 . 



The manner in which the stimuli influence the formation of organs and 

 the nature of their effect may be very different ; in the simplest cases there 

 is a directive influence. We see this in the appearance of roots upon the 

 shaded side and in other features which cannot indeed be altogether sharply 



1 Goebel, Pflanzenbiologische Schilderungen, ii. p. 360. ~ See p. 172. 



