 Fluid flow, as a physical force, is also a difference maker during the development of the heart, and ontogeny more generally, and developmental biologists appeal to physical difference makers, which are understood as factors in producing the morphological properties of developmental phenomena (Forgacs and Newman 2005). A physical causation approach was on display in the late 19th century work of Wilhelm His (Hopwood 1999, 2000; Pearson 2018) and especially visible in the early 20th century work of D’Arcy Thompson and others (Thompson 1992 [1942]; Keller 2002: ch. 2; Olby 1986). This occurred in the milieu of increasing attention to the chromosomal theory of inheritance and attempts to explore developmental phenomena via classical genetic methods (Morgan 1923, 1926, 1934). Thompson appealed to differential rates of growth and the constraints of geometrical relationships to explain how organismal morphology originates. Visual representations of abiotic, mechanical analogues provided the plausibility, such as the shape of liquid splashes or hanging drops for the cup and bell configurations of the free-swimming sexual stage of jellyfish. If physical forces generated specific morphologies in viscoelastic materials, then analogous morphologies in living species should be explained in terms of physical forces operating on the viscoelastic materials of the developing embryo. Yet morphogenetic processes that produce the shape and structure of morphology have been seen primarily, if not exclusively, in terms of genetics for the last half-century. Physical approaches moved into the background as molecular genetics approaches went from strength to strength (Fraser and Harland 2000).