are seen only in one direction as is exemplified in the butterbulb-cup where the number of petals may constantly exceed five (the normal number), but hardly ever, if ever, falls below five.
In some instances the members with a particular kind of variation are found to run to two common forms and those commonest forms are not the mean (arithmetical) form.
The great drawback in the application of the statistical method is that there is often no (illegible text)exclusion by which we can distinguish between variations which are congenital and variations which are acquired, and
^ so no light is thrown on the problem of the origin of the innate characters of species.
Regarding the causes of variation biologists still profess great ignorance. But the experimental work which has been carried on during the past years has shed some new light on the problem. Professor Cosrsar Ewart is one of the foremost workers in this field and some of the results of his observations and experiments are given in a work entitled The Penycuik Experiments and a general summary is given in his address before the British Association at Glasgow (1801) on "The Experimental Study of Variation". From his experiments inof the breeding of pigeons and of rabbits he concluded that age is a possible cause of Variation. It is possible also that the offspring may vary with the condition of the germ-cells at the moment of conjugation. For instance, Mr.H.M.Vernon,
