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The criterion of intermediate forms (also called the criterion of continuity) is used to recognize homology when structures in different organisms appear very different at first glance. It asks: Can we connect these different forms through a series of understandable intermediate stages?
This criterion complements the other two homology criteria (position and specific quality) and is especially useful when those are hard to apply because of extreme modification of a structure.
Basic Idea of the Continuity Criterion
A feature in different species is considered homologous if:
- There is a series of intermediate forms
- These intermediates can be explained as gradual transformations of one and the same original structure
- No abrupt “jumps” or unrelated origins have to be assumed
Continuity can be demonstrated in two main ways:
- Through development (ontogeny) – by following how a structure changes from embryo to adult
- Through comparative anatomy and fossils – by comparing a range of species and extinct forms that show gradual transitions
If such a continuous transformation can be traced, this supports the interpretation that the structures have a common evolutionary origin, even if their final shapes and functions differ greatly.
Continuity in Individual Development (Ontogenetic Series)
Sometimes adult structures are so different that only the developmental path reveals their common origin.
Examples (without going into full developmental detail):
- Certain skull bones in vertebrates can be recognized as the same elements even though, in adults, they are fused, reduced, or reshaped differently. The embryonic pattern of bones and their later transformation provides a “bridge” between very different adult skulls.
- Complex organs that appear “new” in one group can often be traced in embryos to the same tissue primordia found in related groups. The stages between the simple embryonic condition and the complex adult form show how a common starting structure can diverge.
Here, the developmental stages act like “snapshots” of intermediate forms. If two seemingly different adult organs pass through similar intermediate stages, the continuity criterion supports their homology.
Continuity in Comparative Anatomy and Fossils (Morphological Series)
In other cases, continuity is reconstructed by comparing different species, especially when a fossil record is available.
Morphological transformation series
A morphological series is a set of species whose features form a gradual sequence from one extreme to another. For example:
- A bone element might be large and separate in one group, smaller in a related group, and finally fused or relocated in another group.
- Jaws and associated bones in early jawed vertebrates can be traced through fish, early tetrapods, and later land vertebrates. Along this series, some bones gradually change size, position, and function (for instance, bones that originally formed part of the jaw joint become small elements of the middle ear in later groups). The sequence of intermediate forms makes it possible to identify these bones as homologous despite their new function.
Such series can involve:
- Living species alone
- Fossils alone
- Or a combination of both
The crucial point is that the differences between each neighboring form in the series are small and interpretable as modifications of the same basic structure.
Fossils as evidence of continuity
Fossils are particularly important when living forms show only the “endpoints” of a transformation:
- Transitional fossil species can exhibit mixtures of traits that look “intermediate” between two modern groups.
- If we can line up fossil and living species so that a particular feature changes step by step (e.g., size, shape, position, or degree of fusion), then the continuity of form supports homology.
Again, what matters is not a single “perfect” intermediate, but a chain of gradually changing forms that connect two very different-looking structures.
Distinguishing Homology from Analogy Using Continuity
The continuity criterion is especially useful when deciding between homology and analogy:
- Homologous structures typically fit into a transformation series:
- They can be linked through intermediate stages in development, anatomy, and/or fossils.
- The changes can be explained as modifications of an inherited basic plan.
- Analogous structures (independent adaptations with similar function) usually lack such a shared transformation series:
- Their development may start from different tissues or patterns.
- Their internal construction and connections to neighboring parts differ.
- No plausible morphological or developmental pathway links them back to the same ancestral structure.
A structure that looks similar but appears “from scratch” in a different location or from different precursor tissues, without any continuous series tying it to the other structure, is more likely analogous than homologous.
Limitations and Cautions
The criterion of intermediate forms is powerful but not always easy to apply:
- Intermediate forms may be missing from the fossil record.
- Developmental stages can be strongly modified and may not obviously resemble ancestral stages.
- Evolution can involve losses, fusions, and shifts that complicate reconstruction.
Therefore, the continuity criterion is usually applied together with the other homology criteria:
- Criterion of position (relative location and connections)
- Criterion of specific quality (unique fine structure or special features)
When all three support the same conclusion, homology is considered well-founded. When continuity alone is weak, it is treated with caution and checked against additional anatomical, developmental, and molecular evidence.
Summary
- The criterion of intermediate forms (continuity) tests homology by asking whether different structures can be connected through a sequence of gradual transformations.
- Continuity can be documented via individual development, comparative anatomy of living species, and fossil series.
- A convincing transformation series supports a common origin (homology), while its absence often points to independent origin (analogy).
- Because intermediate forms are not always fully known, this criterion is most reliable when combined with the other homology criteria.