534 Alexander Goodman More Scientific Papers. 



of autumnal migration does not lie at all arbitrarily North and South ; 

 but, to a great extent, at right angles to the "Isotherms" of winter 

 temperature, which run in a direction of N.W. and N.N.W. in Central 

 and Western Europe, and migration must therefore take place in a line 

 from the N.E. and E. to W.S.W. and W. With future observation I 

 feel little doubt that the streams of migration may hereafter be laid 

 down in a manner analogous to that of the ocean and air currents ; and 

 that the laws which govern those streams will be found to depend upon 

 the nicely balanced influences of 1st, prevailing winds ; 2nd, physical 

 features of mountain chains and river systems;* and 3rd, the distribu- 

 tion of water and land; as well as by temperature and the all-important 

 consideration of a sufficient supply of food, which are at present thought 

 to be almost the sole cause. It will thus be seen how desirable it 

 becomes, with a view to future generalization, that additional pains 

 should be taken to register exactly all occurrences of our rarest birds. 

 At least the month, and far better the day, are well worth recording 

 where possible, instead of the vague notices of "lately" or "within 

 the last few months," which cannot serve a purpose like the present. 



Another point upon which some further information would be very 

 acceptable is with respect to the periods at which the old and young 

 birds move. I believe it is very generally thought that the old males 

 precede the females and young by an interval of several days ; but that 

 this rule does not hold quite absolutely I have had good proof by 

 rinding female redstarts and blackcaps, as well as males, among the 

 first flights ; and if the females are less noticed they are also more 

 retiring in their habits. I have certainly shot both male and female 

 godwits from the same flock in May. 



And if in most cases "the females and young" are later in advanc- 

 ing to their breeding haunts, is it of males only or of both sexes that 

 are composed those small detachments of old birds only, which, at 

 least in the case of the dunlin, are the earliest to return in autumn ? 

 How comes it, too, that when the birds of this family lay but four eggs, 

 so far smaller a proportion is found among the September flocks, unless 

 it be supposed that most of the parent birds have hastened on by them- 

 selves at an earlier period. Yet sometimes at least the female is found 

 accompanying the young brood. 



May we find some good observers who will try to solve a part of 

 these most interesting problems ! And even if no precise rule can be 

 fixed, why need we despair of discovering something more than is 

 known at present of the general laws ? though Natural History, and 

 especially migration, be not reducible to mathematical exactness. 



* It is believed that the course of rivers and the neighbourhood of the coast 

 are both followed in preference by migratory birds. 



