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At the time, the "preformation theory" was probably the most widely accepted i.e., that the adult form exists in miniature in the egg or germ, development being merely an unfolding of these preformed parts.
To the theory as a whole, because of its fundamental conception of preformation, and to its subsidiary hypotheses, piece by piece, there has been energetic opposition on the part of the upholders of the modern mechanical theory of epigenesis.
From this point of view the process, which, in its superficial aspect, is epigenesis, appears in essence, to be evolution, in the modified sense adopted in Bonnet's later writings; and development is merely the expansion of a potential organism or "original preformation" according to fixed laws. II. The Evolution of the Sum of Living Beings.
This theory certainly contains all the monstrosities of preformation in the germ, the mythologies of the infinitely small, and it suffers shipwreck in ways as diverse as the number of its sides and parts. But it has the merit of clearly disclosing the impossibilities of purely chemical explanations.
The truth is that biological processes are not within our powers of conception as those of physics and chemistry are, and Bateson's hypothesis is nothing but the old theory of preformation in ontogeny.
Modern biologists ridicule the preformation hypothesis as altogether too artificial.
In this I endeavoured to make clear that in reality mechanism is sufficient to produce the organic bodies of animals, without any need of other plastic natures, provided there be added thereto the preformation already completely organic in the seeds of the bodies that come into existence, contained in those of the bodies whence they spring, right back to the primary seeds.
Moreover, he sees, especially in those stages which caused the physical development of man, and which became the material basis of his spiritual productions, moments of development which cannot be explained by natural selection or by a coincidence of material circumstances, but only by the preformation of the body after a certain design and for a certain purpose.
Thus the problem of the “development of form” or of “heredity” was, so to speak, ruled out of court; all that was assumed was continuous growth and self-unfolding. Opposed to this theory was one of later growth, the theory of epigenesis, which maintained that the organism developed without preformation from the still undifferentiated and homogeneous substance of the egg.
A specially energetic fellow-worker on Eimer’s line is W. Haacke, a zoologist of Jena, author of “Gestaltung und Vererbung,” and “Die Schöpfung des Menschen und seiner Ideale.” In the first of these works Haacke combats, energetically and with much detail, Weismann’s “preformation theory,” and defends “epigenesis,” for which he endeavours to construct graphic diagrams, his aim being to make a foundation for the inheritance of acquired characters, definitely directed evolution, saltatory, symmetrical, and correlated variation.
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