GENERAL SUMMARY AND CONCLUDING REMARKS 



It is difficult to do this until we know in the first place what 

 guides the response into certain definite channels. There 

 may even be other factors which might contribute towards 

 the result we see. A suggestion was put forward in the life 

 of the Blackcap that the temporary separation of the sexes, 

 which occurs in the case of many migrants, might lead to an 

 intensification of the response when the crucial moment did 

 at length arrive ; and moreover, that it was almost possible 

 to trace a distinct gradation in the visible manifestations, 

 commencing with the species that pair for life and culmin- 

 ating with those which are necessarily separated at the 

 commencement of the period of sexual activity. A consider- 

 able body of observations taken from the lives of many species 

 would be required to show whether this suggestion has 

 sufficient foundation of fact. Our comparisons, it may be 

 remembered, showed that the reactions may be alike, or 

 unlike, or differ only in degree. No one could tell whether 

 the bird he were observing was a Grasshopper Warbler or 

 a Savi's Warbler, if he were to judge by attitude alone at a 

 time of sexual excitement ; no one on the other hand could 

 mistake a Willow Warbler for a Chiff-chaff under similar 

 circumstances, though he might possibly confuse a Willow 

 Warbler with a Wood Warbler ; and so we might draw up a 

 list of closely related forms showing similar irregularities of 

 behaviour. Of the remainder of our more common Warblers, 

 the Blackcap can be distinguished from the Garden Warbler, 

 the Whitethroat from the Lesser Whitethroat, and the Beed 

 Warbler from the Marsh Warbler. The last two species are 

 well worthy of attention, so remarkably alike are they in 

 appearance, and yet so far apart in their power of song and 

 in emotional manifestation. Well may we be excused some 

 feeling of disappointment with our feeble efforts at inter- 

 pretation. That the difference in the attitudes they assume 

 is one of degree only can be seen by comparing the 

 corresponding plates in the lives of the two species, wherein 

 the wings are represented as partially and fully expanded. 



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