Both in physiology and pathology the study of simple, incomplete, and degenerate forms usually sheds a light, sometimes very clear, on more complex and perfect types. For example, such works as those of Spencer, Maudsley, Laycock, and Carpenter show how much the relation of mind to organisation is elucidated by careful observations of the nervous system in the lower orders of the animal kingdom, and of its condition in the abortive and morbid specimens of the human species. In pathology, more particularly, it is not usually where disease has attained full maturity or has advanced to its last stages that we may expect to find its point of origin, mode of progress, or essential nature; rather it is where pathological change is only beginning, in tissue that deviates but little from the healthy standard, or in function that is but slightly disturbed: so in the special pathology of the nervous system much may be learned regarding the more serious diseases by minutely examining and considering the features of the slighter and less striking disorders.