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Yet a third difficulty is raised by those who ask us to fix our attention on forms of life like Lingula, the lamp-shell, which, though millions of years elapse, do not evolve. If there exists a Law of Progress, they say, how is it that such creatures are exempt from its operations?

Finally, a somewhat similar attitude is adopted by those who refuse to grant that evolution can involve progress when it has, as we know, brought about well-nigh innumerable degenerations. Granted, for instance, they would say, that the average Crustacean is in many ways an improvement upon the simple form of life from which we must suppose that it arose, yet we know that within the group of Crustacea there are several lines of descent which have led to the production of parasitic forms—animals in which the activity and complex organization of the ancestral type has been sacrificed, and as end-product we are presented with a hateful being, an almost shapeless mass consisting of little else but over-developed reproductive organs and mechanisms for sucking nutriment from its unfortunate host. Such a result is revealed to us in the Crustacean form Sacculina, and is paralleled by countless other examples in almost every class of animals. The degradation of parasites and sedentary types is equally a product of the evolutionary process with the genesis of the ant, the bird or the human being; how then can we call the evolutionary process progressive?