680 • REPORT— 1903. 



server found that the younger the seedling is the greater is the influence of external 

 circumstances upon its adult characters, and in the second place that an even 

 greater influence is exerted upon the characters of a plant by the external circum- 

 stances aifecting the mother-plant. If these results hold good for animals as they 

 do for plants, we should expect to find, then, that the external circumstances affect- 

 ing the mother at the time she is maturing the eggs in her ovaries and the external 

 circumstances affiscting the embryo before and during the larval period are of 

 far greater importance in affecting the curve of variation of the adults than are 

 the external circumstances affecting the young in their period of adolescence. We 

 must come to the conclusion, from these considerations, that the general varia- 

 bility of a brood or progeny of a single pair of parents must be very largely the 

 effect of the varying conditions affecting the gametes from the earliest stages of 

 their genesis in the gouophore, the fertilised ovum, and the early stages of develop- 

 ment. We find, however, as I have already pointed out, that some characters are 

 much more influenced by external circumstances than others. Weight and stature 

 in human beings, for example, are probably much more influenced than the colour 

 of the iris or the shape of the fingers. We ma}^, indeed, recognise two kinds of 

 characters, connected, of course, by a complete series of intermediate links, which 

 may be called, for convenience sake, plastic characters and rigid characters. 



Now, in some animals, the characters that are rigid are much more numerous 

 than they are in others. For example, adult salmon or perch are much more 

 variable in size and weight than adult herrings or mackerel ; some species of butter- 

 flies are much more variable in the colour and pattern of their wings than other 

 species ; some species of birds are much more variable in their plumage than others 

 are. Several other examples could be chosen to illustrate this point from the 

 higher groups of animals ; but I wish particularly to call your attention to several 

 instances found in the Cceleuterata, because it was the special study of this group 

 of animals that led to the train of thought I have ventured to put before you. 



In all the sedentary forms of Ccelenterates the mouth is surrounded by a circlet 

 of tentacles. These organs are used for catching and paralysing the prey and 

 passing it to the mouth to be swallowed. They are also very delicate, and indeed 

 the only specialised organs of sense performing a function similar to that of the 

 feelers or antennae of Arthropoda. There can be no exaggeration in saying, there- 

 fore, that they are of the utmost importance to the animal. In some groups of 

 Ccelenterata, however, we find that they are fixed in number, but in others that 

 they are variable. 



In the Alcyonaria, for example, the number of tentacles of the adult polyp is 

 eight. I have examined manj^ thousands of polyps belonging to the suborders 

 Stolonifera, Alcyonacea, Gorgonacea, and Pennatulacea, and I have not found a 

 single example of an adult polyp with either more or less than eight tentacles. 

 This is a character, then, which is remarkably well fixed in the Alcyonaria. It 

 does not fluctuate at all. The tentacles of the Hydrozoa, and of many of the 

 Zoantharia, on the other hand, fluctuate considerably in number. In some forms, 

 such as Tubularia among the Hydroids, and Actinia among the Zoantharia, the 

 number of tentacles is considerable, and it is not, perhaps, surprising to find varia- 

 tions in their number. But in many cases, when the number of tentacles is small, 

 there is also frequent variation. In Hi/dra vi7id{s, for example, the number of the 

 tentacles is 6, 7, or 8, and more rarely 5 or 9. 



Again, in the Alcyonaria, the number of mesenteries of the adult polyp is 

 always eight ; never more and never less. 



In the Zoantharia, on the other hand, the number varies not only in different 

 sub-orders and families, but even in different individuals of the same species from 

 a single locality. Parker found, for example, that the number of non-directive 

 mesenteries in the sea-anemone Metridium marr/inatum, collected at Newport, 

 II.I., varied from four to ten pairs in those forms with the normal number (2) of 

 directive mesenteries, and that there were further variations in the number of 

 non-directive mesenteries in those forms with an abnormal number of directive 

 mesenteries. In fact, of the 131 adult specimens collected, only 40 or about 

 33 per cent, exhibited the arrangement of mesenteries which is regarded as normal 



